Thanks for playing!
I'll look in to the burst not working (You might've just been using it the same frame a bullet hit you?) and the sound breaking. A lot of the other bugs I know about (leaning on walls and restarting being weird in general) but were a more complicated issue to fix.
Safe spots are a problem certainly, the stage 2 miniboss one is pretty easy to fix by just adding a bit to his lower hitbox, but for the stage 2 main boss I'll have to tweak the pattern. The weapons are designed in such a way that even if you get behind a boss you still can't hit it much (the homing shot is suppose to disallow aiming for enemies behind the player) but ideally they shouldn't be able to get into that situation in the first place (though I guess a couple minor ones on easy isn't the end of the world).
Also,the 1-up from the biker is a joke. He only drops it if he pops his wheelie.