It's definitely something else, but I'm giving you games with similar styles of action as reference for my pain points.
Even if we compare it directly to Metroid, the fact remains that the controls are more complicated. The base is as you say, yes, but the game gives you the ability to shoot in basiclly any direction. How other similar games handle it is more simple, you can shoot forward, diagonally up and down, and straight up and down. Ultracore has you able to shoot anywhere in between.
Problem being that it's not a simple matter to move from say, shooting forward to shooting diagonally up towards an enemy, and keep hitting that enemy. You have to wiggle your gun cuz enemies are never positioned such that you can shoot a constant stream at them. Enemies mostly don't die in one hit either, so it's an annoying cat and mouse of trying to get your bullets to go where you want them.