The mechanics in this were nothing short of sadistic. Requiring certain pickups for upgrades is a malicious idea to begin with, then making the player grind for money to buy those upgrades on top of that is frustrating and demeaning. Then, on top of that, the maximum available upgrades still made each level stupidly difficult. You should be able to grind to make your character proficient, not just barely competent.
Even at maximum upgrades, I barely scraped by. There were many levels where missing a single pickup would cause me to die, and others where death was barely avoidable even with every pickup. The gun mechanics were blatantly shortchanged, as every single enemy had better short-range and long-range capabilities and more angular articulation. Even rockets would veer off wildly and not hit their target or fail to catch up with it, even when launched at point-blank range.
I'm not sure the developers got it in their head that rage equals fun, but I admit that it was fury that pulled me to finish this game to its incredibly disappointing conclusion.