Good
Some bugs were already mentioned like the enemy spawning when using the pause button, loosing money when buying, robots still shooting when sphere is destroyed (I had to restart because of that after lvl 8 sphere was destroyed but the game didn't continue).
I don't see any use in the different difficulties since you pretty much have later on the same hurdles and difficulties on easy as on insane setting and can/must beat them the exact same way, it just takes a bit longer. Maybe give us on easy more money or a quicker recharge of the abilities instead of just varying the hitpoints.
As for the comments suggesting to invest in small robots and swarm the enemy : Have fun failing lvl 10 until you're very lucky.
I chose "electric shield", "shield", "health", "dragonflies", "atomic strike", "neutron strike" and upgraded the default weapon and shields to max and had no problem on insane.