The hit zone for the projectile is so tiny, and you have to lead the fighters so much, and the fighters are so erratic in their flight patterns, that your ultragun is virtually worthless...
You know, if we could do this with children's education -- give out rankable game challenges with arbitrary achievements in each of several different areas -- we'd have kids who were ready to go to college by the time they were eight.
Way to get money quickly: The first level of powerups is the "damage becomes money" level. Buy the extra adrenaline card after level 1 and just spam that first level every level after that, using that to grab lots of money.