Fdb's main selling point was reliability. But with the proliferation of good raft/paxos implementations that I think is not that much of an advantage anymore.
To be able to scale, the database would most likely be composed of several raft clusters which brings couple of other challenges: - You need a sharding layer so that the queries can be routed to the right cluster - You need an orchestrator to be able to manage these clusters
It gets trickier as you try to run transactions across multiple raft clusters. You end up needing to implement 2PC.
FoundationDB solves all of the above except for the query engine part (given it is a KV).