With Cluster + Sharding you can have zero(ish) downtime though when you scale horizontally. Messages sent to Sharded actors are buffered if their nodes ever go down and things will just resume as normal.
You can have that with a queueing system, too. As long as you don't ack the message before you're done processing it, the messages that are processing will be sent to the next available client on crash
Sure but the point is it's built into Akka. If you're looking to replace an external queue system, Akka will replace most if not all of it's functionality.