I'd actually love to be proven wrong, Couchbase looks awesome on paper but some Couchbase staff even confirmed this with me here https://forums.couchbase.com/t/persistence-durability-guaren...
Couchbase Server queues writes immediately. The default is to ack once a write is in Server memory, but you can choose other options. See the response from daschl in that thread.
I see what you were trying to get at with your question. I'm not sure we clarified enough what's going on. I'll respond on the forum.