If you max out all of your upgrades, the "Burn 100% of your fuel meter" quest becomes REALLY HARD. Eventually you're just going too fast and there's too much junk in the air for you to not damage your fuel tank and lose all hope of completing it.
Is it me or do heroes become completely useless about 75% into the game. If i let my hero into the battle for even a moment he dies within 2-3 seconds. Also when I try to pull him out of battle the 1 second delay before he listens to the command is usually lethal.