Ok so I have no clue what is a Scratch limitation and what isn't, so I'm just gonna look at this like any other game to remove bias. Ignore anything that's not possible in scratch!
First impressions:
I hate the font choice. 5x7 is the minimum font size that "readable" but it's far from ideal and I had a hard time reading the text (especially some upgrades with "()" in them, they look like "C"). I like a bit of story as the intro, it wasn't too long, nor too short.
I love the artstyle and have no complaints there.
At first I didn't understand what I was supposed to do, it was probably explained somewhere in the story but you should always assume that players don't focus on what they are reading. The best is to explain what they're supposed to do when the game actually starts. Just a simple "kill all pieces" is enough. If we're talking about UI already, then I'd definitely add a counter that shows how many pieces you killed and how many remain on the map.
Another UI related thing is seeing where the enemy can move to so the player doesn't accidentally move onto a square that results in a death. I guess you can say that this is so that the game is harder, but I feel like it's an important QoL feature. If not all the time, then definitely show it while hovering over an enemy piece.
(or maybe I'm just bad at chess and seeing these moves is the fun part idk)
The kill chaining upgrade is insanely overpowered but makes the game so much more fun! I was quite overwhelmed on the final wave and did die unfortunately. I don't think I could have made it that far without this upgrade.
Not really sure what else I can say about the game, it's very linear and most of it is just well made! Of course it's a bit short, more map choices would be nice etc. etc. but I think we both know that so there's no reason to talk about it.
Overall, it's a well made, really short and simple game without any major mistakes. Kinda makes sense since it's using the rules of chess, which is a pretty well balanced game. KEKW