I mean I'm talking to the overly cynical guy who complains about everything. But anyway, I think the purpose of the shmup levels is to provide a break from the normal gameplay. You can't expect them to be as finely honed as a standalone shmup from a Japanese developer. But they have their own unique mechanics and are pretty well thought-out; they're not just throwaway levels.
My only real complaint about the game is that the time trials for each level. They seem unecessary to me, but would add a lot of playtime for completionists.