The point with the acks is, as far as I can see, that the client initiating the transaction has to 1) know when each worker has received the transaction request, and 2) when the first worker has completed it and what the result of the transaction is (failed due to constraints or success).

Point 1 is in case a worker dies before it has durably stored the transaction request.

Point 2 is so that the client can move on.