Ok, it turns out if might have been a skill issue!

A couple of other points:
* It's not immediately clear which weapon the shard upgrades target
* Two different upgrades have the same name
* It's still great fun
Overall, I thought this was a fun demo, with a lot of potential.
The most glaring problem with the demo is the lack of sound effects. I'm sure when those are added, it will pack a lot more punch and will be more fun to play. As it is, the lack of auditory feedback makes it seem a bit... floaty?
I found the player+enemy scaling a bit confusing, and difficult. The chests seem too random, and even with what I thought was a well-upgraded character (3 weapons, some twice, multiple level-ups and shards, 5 artifacts) I couldn't one-shot the basic slimes, and they kept multiplying. It badly affected the FPS, and by the time the potato boss came on screen no strategy was possible as there was just a swarm of slimes and constant jittering. I hope this isn't just a skill issue :D

It was unfortunate, because I couldn't experience the potato boss's unique attacks or learning timings because the game was just unplayable at that point.
So yeah, more consistent scaling and definitely adding sounds will help.
Some more minor feedback - the upgrade details red/green colouring, while "correct" (red=down, green=up) isn't that helpful at a glance to players; for example having a higher cooldown timer is bad for the player, but it's coloured green.
Green should be good, red should be bad, imo; keep it simple for the player.

Another little annoyance was with the chests. When you're in the zone and the bar starts filling up, often it's obscured by other bars and you can't see how far along it is. Sometimes the chest despawned before I could pick it up, while being in the pickup zone! I suggest once the player is in the zone, the despawn timer should deactivate.
BUG: I managed to get overlapping "Defeat" and "Level-up!" modals, which was just confusing. Defeat should clearly take priority here.

BUG: At least on the 3 browsers I tried, on itch.io, the fullscreen button make everything blank and then it was still blank when I turned off fullscreen. I had to reload the page to play again.
I had a hard time using the training ground. The GUI seemed to be cut off (and the in-game GUI was a different scale). Escape worked to close the modal, but overall I found it easier to learn by playing.
One thing I absolutely loved was the weather effects. A very neat feature there, and hopefully one which will expand in the future as different skills and different enemies behave differently in different weather.
Thanks for playing! You make some very good points; it's true the difficulty is a little on the hard side (did you know there are two levels?! I think no one has heard the second tune yet!), that will take some tinkering to get right.
The beam... yeah. Undocumented "feature" you can press < and > to cycle through different displacement maps. It's not as polished as we'd like.
Thanks again for playing!
Lovely graphics, really fitted together nicely.
I'm not sure the positioning is spot on as sometimes I definitely had the block in the right place and it still failed me.
While playing I was concentrating on the bottom of the screen, and just glancing to the top to see my progression in the song caused me to miss a block - maybe the notes counter could be at the bottom?
I did love the bit at the end, replaying the song - that was really nice and a great touch.
Very nice! Feels very slick, the music is well-integrated into the game.
It feels like a nice progression, being introduced to the bass (hehe!) and then getting double jump, etc, it's great fun to play.
The game is polished; nice screen transitions, nothing jumps erratically, it all fits well together
The graphics are very nice, this is really good fun! Good job!
Visuals are very nice and the music fits very nicely; it feels like flying around a Roger Dean poster!
The actual gameplay is extremely limited; there are no real clues so you're just flying around randomly.
Other than sounding good and matching the graphics in tone, the music doesn't seem to have any purpose in the game.
Looks like a really nice concept!
The graphics and audio are the strongest parts, and both work very well together. The music is great.
I found it quite frustrating to understand what was happening at first - insta-death when you are hit by an enemy; and it goes right back to the title screen without explaining anything.
Similarly, there is no feeling of reward for killing enemies as they just disappear off the screen.
The jump mechanics are very "floaty" and feel a little clunky.
I don't think this game would take much, just a bit of polish, and it would be great fun.
Quite fun for a simple concept; love the pixel graphics but the pre-made textures backgrounds don't really fit it.
Easy to play, jump straight in and you're away!
More oomph needed to mark events in the game - hitting, being hit, levelling up - these events should have a bigger audio/visual impact.
I liked the music, it fitted well in the background and added feeling to the experience.