I'm not sure what you mean by not as flexible as relational model. I'll address SSB, as I'm more familiar with it, as I've built a CRDT system on top of it in clojurescript.

Each message can be linked by its content addressable hash. The engine allows for Map/Filter/Reduce queries which are not much different in capabilities from SQL queries, these can even be indexed to speed up views. The only real difference is that no schema is enforced on each message type, but it's not that difficult to enforce something like that on a higher layer when publishing entries.

If you think about it, storing JSON entries is a lot more flexible than table rows. Also, the content stored need not be JSON, but EDN, transit or reference to binary blobs.