Text is REALLY small on the steamdeck but the little bit I played was really good!! I'll probably be playing this in bed for a while, haha.
Edit: I only managed to get past the first boss and a little into a second area before I died, but it started natively with no additional setup/fighting to get it to start. I had to set the controls (since Steam doesn't "know" the game yet) to be the default Gamepad Template in-deck. The only issue so far was a big drop in frames while it was loading the very first upgrade set, but all the ones after, and the boss trigger, were fine.
Got an almost 40 minute run, 64 powerups, 6 bosses beat, little hiccups. Runs the massive waves pretty solidly!