I try to push Mumble over Skype, the main issue with Mumble is that it's not as easy to get into. You spend too much time configuring your client, finding a server, etc. Things that should be fairly trivial. Then you may spend more time figuring out why you're not being heard, or why nobody's hearing you. Mumble is great but there's too much friction involved. I prefer Discord[0] because it's so easy to use by comparison, to test it you don't even need an account, you just run it from your browser. I just wish Discord had a Linux Client.
Right. And furthermore, Discord is free and totally propped up by investor money. Which means the day will come when the experience is totally ruined in an attempt to make money, as fast as possible.
Of course, I still willingly use Discord despite this - because it generally just works (even in browser) and has an extensive feature set, and is relatively easy to use. (this is among friends who aren't technical, so ease-of-use is a large factor). Previously I used Mumble, but indeed, it's a bit of a fickle beast for things that should be fairly automated (like configuring your voice inputs, etc).
[0]: https://discordapp.com/