When hosting and using their free notifications service, you're basically using their test server with no uptime guarantees. I agree that there's only so much you can expect from a free service, but unreliable notifications make a chat app 100% useless for any serious work.
If you pay, it's $10/user/month for the basic service. Where I live that's about 3x Ms Teams, 2x Slack. And you still have to self host.
Self hosting the service has the same issue as Rocketchat: you need an Apple dev account, a firebase account and an endless amount of free time. It's going to be far harder and more time consuming than just hosting the app.
We haven't had any issues with the free notification service. They did release the code to host your own but I think you'd also have to rebuild and release their mobile app to use it, and that sounds like a massive hassle.
I'm using it for a nonprofit, so $10/month per user is out of the question. Self-hosting and open source are critical to us also.