> a big benefit of that that full stack developers, will be able to communicate between web app and backend services in the same way that services talk to each other in a secure and reliable way
In practice, I have not seen that manifest as a benefit. Services have a dramatically different environment than edge devices. Tools built for services (NATS, Kafka, gRPC) do not translate well to the edge. The latter is used by a group of people who don't care or understand the edge-edge-cases: when a user drives through a tunnel and is disconnected, or when they restarts their device, or is throttled by an OS, etc. One issue I found with grpc-web (the alternative to grpc that supports browsers) is that it's severely limited by connection count by the browser - making streams completely useless). Also, grpc-web is neglected by Google.
NATS does not look ready, and is not designed for use, in browsers or mobile apps. It's not a use case that Synadia/NATS care much enough to even mention on their website.
> (react + flutter) in my case
How are you using NATS in Flutter? Using the client library that hasn't been updated in 20 months, with no link the repository and 4 upvotes? Or writing your own custom library to connect using websockets. If you use websockets directly, you'll be writing extra code to handle disconnections, retries, qos, etc.