Nats is not a replacement for pulsar or rabbitmq. It is a message passing system designed to pass lots of messages live, however if nobody is their to receive them they are lost and gone forever. There is a streaming layer but that is closer to Kafka and still does not provide the typical message model with an ack/nack API.

I have used nats in several different ways but since it can be lossy its never been considered as a replacement for a pub sub message queue on my end. We used it for a chat message layer and that worked pretty well.

As for its message passing layer that can be interesting but you end up writing all the retry and failure logic anyway so its usually just better to use an existing message layer that handles all of that for you anyway without all the funky abstractions.

Again, its interesting but nowhere near close to being a rabbit or pulsar replacement if reliability is a goal.