I've used both Firestore and MongoDB's Realm Device Sync. While Firestore works great in terms of scaling, the performance and speed for client side write is far behind realm, especially when client side is doing some batch writes.

Realm uses techniques like data compression and delta sync to reduce the data size when uploading (https://www.mongodb.com/docs/atlas/app-services/sync/details/protocol/), I'd like to see Firestore doing the same and improve the performance of client side batch write.

Also, what I've noticed is that Firestore's client side write performance degrades a lot after some time being offline, this isn't great for apps that would like to optimize offline experience. Realm doesn't have this issue.