A big part of most shmups is complex scoring systems which isnt something you can understand by just looking at them. I think thats why Ikaruga has more mainstream popularity, the polarity switching is easy to understand just by looking, a lot of ikaruga fans dont even think about chaining.

Bayonetta has the same thing as Ikaruga where witch time is an easy to understand mechanic for casuals while the bigger mechanic the game is designed around is dodge offset. But an ideal game would make complex mechanics easy to understand for casuals as well.