Major Description
WaitUntilReadyAsync always fail with Cluster level WaitUntilReady is only supported by Couchbase Server 6.5 or greater if the first node in the comma separated list in the connection string given to Cluster.ConnectAsync is unavailable. I expect it to try the next nodes and if all nodes fail retry (or at least give a more relevant exception).
ConnectionStrings should failfast: "couchbase://,localhost" or "couchbase://localhost1,,localhost2"