Firestore's subcollection feature is pretty cool. As far as NoSQL dbs go, it's flexible query and indexing make it more suitable to app development than, say, DynamoDB.

To me, the lack of good developer tooling has been what holds it back. Not having a maintained type-safe querying ORM/ODM library like eg. Prisma or Mongoose is a major flaw imo (not to mention missing out on the coolest firestorm)