I wish i knew about this Demo sooner, i just tried it out as all characters as i have already beaten the first Udongein X as both reisen and seiran on lunatic and really liked the first one aswell and left a review for it too so i want to share my review and feedback about the demo so far.
The graphics are a huge improvement, very smooth and very readable even if it completely dropped the "Megaman X" kind of style that the original was trying to go for, but looks better so no issues on this department.
The scanlines filter i feel needs some adjustments maybe give some transparency to the black scanlines so they are not completely black or for reference look at how the Megaman X8 Demake handled it, they handled that perfectly so i think that game is a good reference all things megaman X, only problem with the scanlines in this one is that depending on the resolution the scanlines will become really pronounced , the black lines will be too visible if you record it or resize it in anyway, i don't know what they did in X8 16bit Demake to prevent it but maybe copy what they did there as i found no issues when recording the scanlines or resizing the window.
The animations and everything visual looks good except for some things that i think actually hinder the gameplay which would be invincibility frames flashing effect (which i will talk about later) and the very miniscule Health bars for both the player but specially the bosses, i think its way too small, when its bigger it helps to quickly be able to glance at it while you are dodging bullets, when its small you have to hyper focus on that small area and that's distracting while you are against a boss, in the original the health bars are big enough that you don't have to glance "too far away", also , the following is a minor issue but not sure how i feel about the health bars not being divided by small bars anymore since this is also present since megaman X4, thing is that when you have 3 different characters its harder to compare how much damage each attack does when there's not an intuitive way to know , for example, we all know that the charged attack as reisen is supposed to do 3 bars of damage and 1 bar with a normal shot, but as Ringo or Seiran honestly you can't tell and i feel its specially more important for Ringo because shes the one who attacks the slowest as she only has one bullet on screen so you gotta know if level 1 shoots are word mashing against a mini boss (if the damage is 1 then its probalby way too weak) or if its better to use level 2, but its hard to know because its hard to know exactly how much the damage difference is exactly (specially because mini bosses as a rule don't have a visible health bar but you would know how much damage you are doing by testing against a boss who does), my suggestion would be to have the health bars tell you more, it can be with bars or numbers or have the damage information somewhere in the game now that there's this in-game encyclopedia in the game, that would be good too, for example telling you how much damage each ringo charge does and how much the damage buff for Seiran is, for normal enemies this is not important but for Bosses for sure i think it is.
Now about the invincibility effect, i don't know if its the darkness in the kokoro stage but for the life of me i couldn't figure out how long invincibility frames for bosses work or when its triggered, what didn't help is that the sprite flickering effect that lets you know they are on invincibility is hard to see for some reason, not sure if it was because of the scanlines or the dark room but it was hard to tell compared to the original game.
Now about gameplay related stuff:
What i couldn't wrap my head around are the rules on Boss invincibility frames, as Reisen when you hit a boss with a charge 3 , they go into invincibility, that's normal so far, but when you hit with a charge 1 apparently that doesn't trigger invincibility until you do 3 consecutive ones?, which is fine but what i couldn't understand also was the length of the invincibility, it seemed incosistent, seemed to vary, seemed like the charge 2 made them flicker for less time so consistently attacking them with charge 2 would be the most efficient way to damage them just like it happens in X5 and X6, as Seiran i couldn't simply tell how long invincibility lasts for, seemed much longer than usual but it was an issue because it was hard to tell when invincibility ends and that will make it so the boss will do contact damage to you while they are on invincibility when you try to hit them thinking the invincibility wore off, and because understandably they cannot be stunned when they are already doing an attack the invincibility itself will often wear off after the second opportunity to attack them goes away slowing down the fight, if the intention is so bosses last longer as seiran thats okay but i would like to suggest something to prevent bosses to ramming you with contact damage when you are trying to hit them when they are on invincibility, maybe make a rule where they cannot do contact damage when they are on invincibility or make it so your own melee attacks as Seiran are immune to only contact damage from bosses and enemies (with the exception of melee attacks from enemies or any bullet based attack) which would help in general for the rest of the game to avoid hits that feel like cheap shots as this was also an issue in the original aswell, i understand this would make it abusable but to prevent that make it so the rule only works during the active frames of your melee attacks and not the recovery or startup, give the contact damage invincibility only during the active frames of your melee attack basically prevent "trading hits" in the fighting game sense, you are a fighting game guy so i am sure you know what i mean, that alone i think will make it more fair.
Now about Ringo, i like the idea that she's not outright Axl , this detonation gimmick its original and i like it a lot , specially that she can aim in many directions, only thing that i found strange is her Bomb, seems very underwhelming or very situational, i assume it gets better with upgrades later in the game but in terms of risk vs reward seems like every single time it will be preferable to use the 3 bombs to heal rather than the very situational single bomb, so far with Reisen and Seiran the bomb have a very straight forward uses, as Reisen to deal with strong non-boss enemies and minibosses faster, and as Seiran for covering ranges she can't reach and also to kill some big enemies slightly faster such as the UFOs in the first game and in this one both Reisen's and Seiran's help to kill lily faster aswell, but as Ringo i dont see it being very useful when the healing bomb exists, the strength i guess comes from you being able to detonate 2 shots at the same time on screen but it would be only useful if there's a wall for that shot to be attached to in the first place which i imagine it wouldn't happen too often and on bosses i am not sure if the second explosion would get ignored by the invincibility triggered by the first one, maybe it would be more useful if the bomb one already started on level 3 or it attaching to enemies, or something to make it compare to the bombs of the other 2 characters, maybe i have no idea how to properly use it but i couldn't figure out a practical way to use it, i think there's a lot of potential for ideas there but so far its doesn't seem intuitively useful
About the general feel of the game, movement, such as walking, dashing and jumping feels way "heavier" , coming directly from the original Udongein X doesn't feel all that good compared to that game, the original is very snappy but this one feels as if you are under water, i prefer how the first one feels , this one feels too heavy in comparison , much slower than any other megaman X clone i have played, try reaching out to the developer of X8 demake as that guy is basically an encyclopedia of how megaman X feels, he nailed it and knows some tiny details that are easy to miss, such as the 7 pixel walljump present since megaman x1, this basically makes it so you can walljump/wallkick without requiring you to actually be touching the wall (the sliding on wall animation) but let you wall kick as long as you are close to it enough, basically being able to wall jump by just pressing jump when you are close enough to a wall in the air (without holding a direction, just pressing jump gives you a wall kick), its easy to test in megaman x, stand near a wall without touching it, neutral jump then press jump again without holding a direction and you will wall kick, would be nice to see that in this game as its part of how megaman X feels.
I couldn't test this but also when dashing, if you hold crouch make sure that after the dash animation is finished it transitions directly to the crouching animation instead of going directly to standing then crouching, this is to avoid the standing hurtbox in between dash and crouch so you can dodge certain attacks consistently, this is an overlooked issue in X5 so i assume its here too but couldn't find a good way to test it
About the level 3 bomb for all characters i think it would be a good idea to include an option have a dedicated button for that as holding the button really lends itself to accidentally use 1 bomb, holding it is fine but would be nice to have an additional button that can does that.
While holding the bomb button i think it would be nice to have some kind of visual charge effect that lets you know or reminds you that this mechanic exists, maybe like a circle that closes around the character which would simultaneously also let you know when it activates
About the Music and sound effects:
The music is great, a huge improvement even if it lost the megaman X feel but i guess it fits the new sprites the game has so no issue there.
The hitting and explosion sound effects though feel very "bland" or "soft" for the effects that matter the most, this is hard to explain , feels like there's an echo in every sound effect and makes it sound so soft you don't feel the hits , doesn't make you feel like you are hitting something, maybe raise the default volume for some of those sound effects, the same for the getting hit sound effect feels like it should be louder by default, the sound itself when enemies die i think its the weakest one, not very satisfying.
The other sound effects are ok such as the ones for shooting and jumping but i think for most of the sound effects the individual volumes feel a bit weird, the jump sound feels too loud compared to other things that i think should be louder such as all the hitting and explosion sounds
I normally don't pay this much attention to sound effects but in this case it caught my attention that much as its very noticeable
The menus all i can say is that i found it a bit unintuitive that you have to press up or down at options that looks like you would have to press left or right.
Would be nice aswell to being able to select your character before the game begins.
With not much else to say i think its going great and looks very promising , i am really looking forward to it and i hope it keeps the best things from the first game as some things don't need fixing, also having steam achievements on the first day of release that would be great too, it really does help to increase the playtime of players, i think it did for the first game although these got added later.
I hope you can see this feedback on time and hope it helps ,looking forward to the finished game, good luck with development!