What do you mean by "scalable without restriction"?

I understand scalability by being able to handle more traffic through adding more nodes to your cluster. But if you want to handle more traffic, adding more replicas to a single Accord replica set won't help. Any two (fast path) quorums must intersect. And that intersection point for two queries is where you need to spend resources to handle both queries.

So - unless I'm missing something here - in order to scale out to be able to handle more traffic, you must shard the data, there's no way around it. Even assuming 0 failures won't help.

And once you shard your data and use multiple Paxos/Raft/EPaxos/Accord/whatever groups, you lose strict serializability across the shards.