I think what makes this game hard for me is that the game has the ability to dodge roll and clear the screen of bullets, which compared to binding of isaac, allows for a lot heavier bullet hell experience.
I think giving rewards based on not getting hit is actually a good thing in this game since its to reward those who play well, similar to how in BoI if you received red heart damage you pretty much couldn't get into the Angel/Devil room without a teleport for that floor.
It can use a bit more, that's for sure. as with what everyone else said, I do think shields should act as soul hearts in BoI where it does not affect any of the deals, or in this case item drop chances (coolness %)
Also The scrambler is pretty neat. Good power, tracking bullets with long life time and range. allows you to easily kill enemies from behind cover.