A 3.6 mongod with FeatureCompatibiityVersion set to "3.6" cannot connect as a client to a 3.4 mongod. A message about wire protocol mismatch is generated.

Server min and max wire version are incompatible (0,5) with client min wire version (6,6)

As a result, the db.copyDatabase(), db.cloneDatabase, and db.cloneCollection methods fail when run on a 3.6 server with FCV "3.6", and sourcing from a 3.4 server. We suggest adding documentation on this point, that these methods require that the target 3.6 server have FCV "3.4".