Too easy; and then too challenging
The game was fine for Easy and Medium, even a tad too easy. Then came the Mr. Smith and Ninja levels on Hard. Why does Ninja magically rake in a life every now and then? Is that 'challenge'? I don't think so. It's just frustrating and unfair. And I can't remember the amount of times that I had Mr. Smith beat, and then lo and behold, getting raped 4 times by the jet lets him win, yet he's NEVER in the way.
Make the AI more fair, and then we'll talk.