Quite an ambitious project! It took me a while to get used to the game's idea of progression, as I was initially pretty disappointed that I kept losing my upgrades. However, I have to admit that getting my first "permanent" upgrade was a big game changer and made the initial frustration worth it!
The environments were definitely barebones, but I think in a way they nailed the early 90s 3D look, and the desolate atmosphere grew on me over time. The music was a big help there, especially the "epic" theme playing in the later part of the game! I quite enjoyed the dark visuals of the "peak" area, but the green of the "poison" areas made it a bit difficult to see where you were supposed to go, because everything kinda blended together into one green mass. It also wasn't really clear to me which areas were meant to represent the past, uncorrupted world, aside from the bright golden temple(?).
The enemies were annoying at first, but I eventually found them manageable, especially after I got into the habit of sucking them with the grapple. I actually found the buzzsaw enemies to be the easier of the bunch, as it was easy to outrun them or get them stuck on ledges. The flying insects were much more annoying to shoot down, and the golden variant seemed particularly persistent in chasing me down to the end of the earth if left alone. In general, I didn't get much use out of any of the alternate weapon modes beyond their special abilities. The electric shot would have been nice for the flying enemies, if it didn't run out of ammo so quickly.
Some of the platforming was pretty challenging, though I think it was less so due to the platforming itself and more due to the scarcity of checkpoints, limited breath duration in toxic pools, and the long runbacks from death/falling down.
It took me a good couple hours and a lot of frustration, but I finally managed to climb the peak and get the last permanent upgrade. I was so caught up in the moment, with the epic music suddenly turning into silence and the glorious overview of the level, that I thought the game was finished and almost quit right there!
Fortunately, I remembered the red keycard from hours ago and didn't miss out on the real ending of the game, according to which I had found 4/4 cores, 3/7 expansions, and it took me 2h 31min to get there. (It definitely felt like longer to me, maybe it didn't count the time lost due to deaths?)
As for criticism... I guess if the goal was to make a challenging FPS platformer, then most of the grievances I had throughout the game could probably be considered features. (And I'd definitely be a hypocrite for criticizing a game for its difficulty.) That said, 3D platformers (and especially FPS ones) have been and probably always will be controversial to some extent, and for that reason alone, I think the game could have benefited from a few more checkpoints, as well as more generous timing for underwater breath and the various electric pillar challenges.
Overall, a very substantial game that took some time to get into, but it was absolutely worth it for the atmosphere and the satisfaction of reaching the end!