Hi! Glad you liked it!
The shockwave appears only when the asteroids destroyed had a high speed (I wanted it to look like a more powerful explosion).
Yes you're right about controls, you need to get a bit used to it, it would work better on a joystick that's true, unfortunately I didn't have time to adjust the controls for a joypad but I'll probably do it later to complete the game :)
Leaving it there is also an issue I agree, but you can't score more than a certain amount of points because asteroids can spawn on the edges of the screen and you can't catch them by just standing there, so if you want to score more points you're going to have to actually play :P