Hey Nolan, I'm the game and leveldesigner of Tiny Thor. Thanks for the detailed feedback and your assessment of the game within the genre seems level headed and positive :)
About the two points you've elaborated on:
1) The heart mechanic is definitely unfair at times, as it very much depends on the environment if its retrievable or not. The full game will have some puchasable upgrades that make it easier to get it back / increase the time etc. There's also a late upgrade that allows your hammer to retrieve it. You commented that enemies and the hammer itself seem to push the heart around, which is not correct. The heart just has a logic that makes it bounce away from the player, that bounce must've happened the same time as an enemy "collided" with it.
2) The boss currently has a bit too much HP after we moved a hammer property away from being default and into the upgrade shop mentioned above and didn't adjust its HP. So its definitely too beefy right now. Other than that, the boss and the whole game are meant to be on the more challenging side, but maybe it ramps up too fast here.