Impressive demo overall. The level/environment design is great, exactly what I'd want to see from a premise that feels very much like a mix of Armored Core, Valkyria Chronicles, and just a touch of Ace Combat. Movement feels pretty good, though I'd agree with some of the other comments about lack of 'weight' and there not being much of a reason to switch to the alternate boost mode. Ranged combat works well for the most part. Dealing with the sniper felt a little hit or miss as to whether I could successfully dodge at the last second (which feels great when it works) or get tracked by the shot anyway. I like the inclusion of flares to defend against homing missiles, although I would have preferred to have something in the UI to indicate when they were recharging (assuming I didn't miss seeing it). The music fit the action and was enjoyable to listen to. I'd have liked to hear some additional tracks to fill in the silence between larger fights, but (along with some of the other aspects like the voice acting), I won't comment too much on things that don't seem like the central focus of the proof of concept demo.
I think the biggest critique I have is that the central combat flow of switching between ranged and melee combat kind of falls apart when the intensity ramps up, particularly during the boss fight. While I understand the appeal of having an in-depth, dedicated system for both ranged and melee combat, the fact that half of the combat actions are exclusive to ranged/melee modes leads to a lot of awkward switching between stances under pressure, especially at the intensity level that the game seems to be going for. Needing to take a moment to switch the main cannon with your melee weapon makes sense, but it's frustrating to have to stop shooting and change stances every time I want to block incoming fire or parry, or need to break out of melee just to use my shoulder weapons. The boss doesn't even seem to have this restriction in the second phase of the fight, which leads to a lot of unfun moments of trying to time attacks during melee exchanges and eating a bunch of missiles at close range. I also found the weapon art to be extremely difficult to use during the boss fight, since it takes so long to start up that even activating it right after a stagger opening would always lead to it getting interrupted anyway. The stagger openings themselves felt very short relative to the amount of damage (or lack thereof) that your weapons could do on short notice, to the point where it almost didn't feel worth trying to push for them against the boss.
My take is that it might help the overall combat if dealing with incoming melee attacks was less restrictive, particularly regarding the tracking (so that you have the option of dodging more reliably and aren't required to switch stances and spam parries up close) or if you had some way of accessing your block/parry from any stance. More fluid melee combat could also make it more of a viable option when fighting multiple enemies at once, since I found that trying to engage in melee in those situations would usually just get me shot by the second mech unless I had a huge opening (like getting one of them stuck on terrain or behind a huge building). Heat management during melee could use some attention, as others have mentioned - it either doesn't pose a problem at all when all you have to do is parry (although that gets old pretty quickly), or instantly cripples you as soon as you have to worry about dodging guard breaks or getting shot.
There's definitely a lot of promise here, though. The bones of the movement and combat are largely solid, so I could see most of these issues getting ironed out over the course of development and testing. I think the thing I'm most interested in seeing is how squad combat will fit into this, as well as how much the weapon customization affects the flow of combat. I'm excited to see where this project goes - good luck with development!