1. Are you hosting your Jabber server yourself or are you using the hosted Snikket instance?
2. Does group texting work? The FAQ says it's in beta. If so, how does it work? Is it just an MUC?
If any other JMP users see this, please chime in.
The only unreliability with calls that I've noticed was traceable to the unreliability of my own internet connection. I've confused incoming calls by simultaneously making and taking calls from the computer and the phone, but I think it's understandable that problems might arise and that's not a real use case for me. Once or twice I did not receive a text transcription of a voice mail, but the support is usually quick to address things like that.
I host my own XMPP server and have for a good decade now, and that's what I use with jmp.chat. I can't speak to the quality of their hosting options.
Group texting works fine for me if one of the other parties initiates the group text. I haven't tried to initiate my own group text in well over a year; last time I did, it didn't work. That may or may not be a problem for you, and it may or may not have been fixed by now. Worth investigating more if it's important. I should also say I've only ever used group texts with 3 participants, and can't speak to what happens if there are more nor whether there are upper limits.
Group texts don't use MUC. Rather, they use a special syntax in the JID, something like "+1XXX,+1YYY,...,+1ZZZ@cheogram.com", where the + and , are required, the XXX, YYY, through ZZZ are the phone numbers (no dashes or other special chars just digits), and the cheogram.com at the end is required.
I recommend the cheogram app if you're on android. It has a lot of nice features on top of the Conversations base. I use gajim on my (linux) computer and it works well with jmp.chat.
I'm happy to answer other questions if you have them!