CouchDB, see: https://news.ycombinator.com/item?id=22175530

RxDB:

- Doesn't guarantee convergence. It's up to the developer to provide a feed of deltas to the client and ensure they have the correct effect when applied to the client. This is the one of the hardest parts!

- Provides no help with conflict resolution. Push replication just pushes the new version of each document. The server must figure out how the document differs from the stored version and what that must mean in terms of logical operations.

- Implemented in JavaScript. Difficult to use in native apps.

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I'm not up to date with Gun (it has gone through many iterations), but I think it is currently trying to be a masterless system which means the same concerns as with CRDTs apply.