Indeed, async continuations are not yet supported. Note that async continuations are not part of the BPMN spec (unless you use a sendTask/receiveTask to mimic it), so thats the reason why we didnt prioritise it.
I think, that it is very important for production use. Beside failover, HA, etc. you also need guaranteed flow execution. When some process states are not persisted there can be potential risk of state repetitions.