Shooters don't really feel hard to me, just cheap, unorthodox or require concentration.
If I haven't played Thunderforce IV in a long time, I can still breeze through most of it, but there are a few cheap spots which you have to experience to know how to avoid.
Ai Cho Aniki is very easy, but can be difficult to control if you jump in without reading the manual. Traditional shooters feel natural to me and are kind of like you learn one, you learn them all. Non-cheap games which don't play the way most do are more challenging or require more concentration from me.
By concentration, I mean there are two types of shooters: the ones I can play while half asleep and out of practice or not really paying attention (like zoning out while driving a car)... and the ones where I have to consciously concentrate. Of those, the games most people say are impossible don't usually seem that different from others which simply require me to try.
Aeroblasters isn't that hard of a game. I finished it in one evening as a rental bitd and asked for it for X-Mas. It will always be what I think of as a normal balanced shooter. It just feel perfect to me. The final stage can be tricky and is a bit cheap, because you have to learn the layout to consistently zip through it. The fourth stage boss can also take you out before you figure out how he plays. But otherwise it's simply a matter of remaining focused.