Mumble has a bunch of issues, the main one being their confusing UI, especially on mobile clients. I'm regularly in mumble conferences and for example accidentally switching the room instead of pushing the push-to-talk button happens quite often.

There's also a bunch of more technical problems. For example as mumble uses a udp protocol it handles dropped frames badly, like on a spotty wireless connection. Results in missing audio. Not just that sound doesn't arrive (in both directions) but it also doesn't tell you something's wrong.

Mumble has problems with changing audio setups which require a restart of the client.

Also no video, no simply calling people - you need to go to a server and they need to go there, too. Basically stopped innovating some time ago and everone else moved further.