Here's an unironically good Steam review if you want to go in depth, so I'll try to avoid tl;dr:
It's an IL game where you play one stage at a time, get semi-random waves + a boss and buy cards between them. At the end of the stage everything you bought is taken away and you get booted back to stage select. You unlock starting cards by replaying stages over and over, and as the stages get longer there isn't enough wave variety to keep up, so it gets repetitive very quickly. There is a longer marathon stage but it's got nothing new to give and by the time you've unlocked it you've probably had enough of everything anyway.
Bosses are heavily truncated. It takes until stage 5 for them to get more than one spell. All the boss patterns are set, same every time, which is a staggering missed opportunity when there's no reason they couldn't have a random selection of patterns. The boss selection itself is totally random; if you're looking for a specific boss's unlocks, you need to replay the stage until you get it.
There's no (real) scoring or difficulty levels. There are no non-gameplay aspects that are going to prop it up either. The title theme's good, rest of the music's alright, nothing inspiring. There's nothing to bring you back after you've played it once.
It's the classic procedural-element-game mistake, to compromise so many of a genre's strengths - in this case tension, depth, replay value - and get so little out of it. Still, it can be briefly entertaining when the pieces all fall in the right places, so it's recommended if you're determined to play a Touhou game and the only other option is Violet Detector. If it's random-yet-abusable gameplay you're looking for, play PoFV instead.