Rocketchat uses it as a way to funnel you into paying. You'll get a low number monthly for free. They say 10k month but with a team of 5 people lightly using it we used that in 4 days. Or you can do it yourself, but you'll need to register your own version of the app in both the Apple Store and Play Store. For Apple apparently this is close to impossible so I didn't try.
Nextcloud runs their own server for free, but you have to accept that you'll be sending data via their server. But I've spent the last week hacking away at setting up a Nextcloud in my spare time. Got it mostly set up in a day, then the rest of the time has been trying to get notifications working on mobile. Still not working.
Odoo, I don't think I ever got notifications working.
I have researched other apps: Mattermost does something similar to Rocketchat, using notifications as a sales funnel. Element is similar to Nextcloud, they host their own free server, although I think you can self host that too.
From this experience, I would never try a new app until they have this feature solved, clearly documented, and with proof that it works and isn't a sales funnel.
Can you elaborate on this? I manage a Mattermost instance and there are some features missing from the OSS self-hostable community edition, but notifications seem to mostly work, even on mobile where notification delivery does rely on their gateway
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.
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.