Touhou Nil Soul is a boss rush RPG spanning 11 missions. It's always your group of girls moving on a grid against one powerful stationary foe. At the start of a fight, you have no ammunition (reiki) for your special skills, so you build that up through standard attacks. Enemy danmaku moves around the grid as well and healing and reiki pickups appear periodically, so movement and positioning does have a point. The whole system seems simple at first, but complexity increases to a satisfying degree as your roster grows and gains synergy.
Next, I've found the loot system to be pretty exciting and motivating, reminding me of finding and gambling items back in my Diablo II days. No equipment is just given to you, everything is in the shop. You always know the type of item you're buying, but never its bonus stats. Items of higher rarity cost more and tend to have more bonus stats, but may even come with up to two stat deductions. There's always that risk that an orange (highest quality) gear piece may deduct something important like attack range or movement speed, turning it from a potential winner into something atrocious.
Moving on to the probably biggest weakness: The story is so-so. It's mostly a retelling of Touhou 6, with the twist that through the Nil Soul relic, Reimu and Marisa can create puppet duplicates of who they beat in combat. A sensible idea, as most of the cast could hardly be logically persuaded to ally with them against Remilia. The English translation ranges from solid enough in most cases to pretty bad in some. Especially Sakuya's chapter stuck out to me as pretty butchered, which is a shame, particularly because the overall scenario (Reimu being angry at Sakuya for allying with youkai against humans) seemed to be among the more interesting plot-wise. Most gameplay tooltips are at least accurate and easy to understand.
The majority of missions have a cute action-based minigame that is optional to play and will increase the money you gain after battle. If you don't do well on it, your team leader may start the bossfight on low hitpoints, so it might be smart to skip if you're not confident and the mission is particularly hard. If you're having trouble: I've had the impression that it's easier to anticipate the obstacles by ear rather than sight, as they all pop up with distinct noises.
When it comes to size, the playtime isn't too huge here, but chunky enough if you can't get enough of it. If you only want to see how the story goes, it's over very quickly and quite easy. Folks have called the first playthrough a tutorial and I agree. With 5 difficulties (Easy, Normal, Hard, Lunatic, Extra) that behave like a New Game + each time, the total of 55 distinct missions isn't too bad.
Overall, Nil Soul is a pretty innovative and solid RPG. I've had a good time with it! Within the Touhou fandom, it isn't among the top dogs like Labyrinth or ADiA, but still places in the respectable league of something like Genso Yonbai Ken 2 or Three Fairies' Great Journey.