Guess :V
I see Shoot the Bullet as ZUN doing what he excels at: making beautiful and interesting patterns. Due to the structure of the game, there can be more unintuitive cards that require a creative approach to capture. A card like even 2-5 could never show up in, say, PCB, outside the extra stages.
I've only played three Touhou games: PCB, StB, and now SA. And playing through SA, it's amazing how much more creative ZUN has gotten with bullet patterns; bosses tend to move all over the place, and the game seems to demand more versatility. Perhaps I'm talking out my ass due to limited experience, or perhaps I'm caught in the comparison between old familiar danmaku and new shiny danmaku, but I do think there's a noticeable difference over the years, and I think Shoot the Bullet's experimental patterns have a lot to do with that.
I mean, some of the levels may be frustrating as hell (I still don't know where I'm going to get more successes, except perhaps from 5-1 or various Sakuya nonsense), but most of them are interesting.