Main reason is that it's not in the standard release yet. This means it's more effort to get a build that supports it (build it yourself essentially), and it's not tested as well.
Additionally, most current tools will not be able to read database files using WAL2, although this is not an issue for most applications that only use it for internal storage.
They use CRDTs for syncing, but their site has virtually no details on what data structures are supported. https://mycelial.com/docs/get-started/quick-start