a simple idea well executed! i didn't encounter any bugs on my playthrough.
It could be fun if you could also hit enemies as the harpoon came back down. That plus adding variable speed for different rows of enemies could create fun moments where you get multiple enemies with one shot, maybe if you shoot while moving the harpoon could maintain some directional velocity and hit a couple in a row diagonally.
just some random thoughts I had while playing it, great job!
-Josiah