- Can't host your own server
- Reliance on discord to keep their service
up and running
Arguably more reliable than a private user who could crash/lose internet any time.
No one hosts their mumble server on a private computer in their home. Most are hosted by game server companies or on a VPS which I am willing to bet have at least the same, if not greater, uptime than something like discord. They are also proven, whereas this is a new product that could just shut down at any time, as many similar projects do.
- Not open source
How does that make any difference whatsoever? Why care?
Well, there are moral/ethical grounds to prefer open source projects, but there are also technical considerations to be had.
The open source nature of mumble has led to increased ability for customizing installs and deployments, and the exposure of the complete mumble api and backend through murmur ice RPC. You can do some seriously cool stuff with that, stuff that may not be available for discord ever.
In the event that discord has a market failure or heads in an entirely terrible direction, we have no recourse except to switch back to mumble. If the mumble developers go off the deep end, it is literally a branch away from being the project of more sane individuals.
The community can add features/request features and they are easier to implement. I am not sure if TF2 has ever done this for mumble, but I have submitted many bug reports, some of which ended up getting fixed. Discord, as a proprietary software, is locked into their own development cycle where there is a payoff between adding new features that may be incomplete or buggy to entice users vs stabilizing their platform and focusing on the essence of what people use it for.
- No privacy opt out of data collection
Only valid complaint, but only if you actually care about your browsing privacy while playing tf2, in which case, why? There are also simple tools to prevent it from getting anything.
This reeks of the "I have nothing to hide" fallacy of data security. Remember that, if you are not buying the product, you are the product. Free discord users likely factor into their profit model somehow, and that is entirely up to them how they manage that. You are technically correct in the statement that, probably almost all of us have nothing worth hiding from discord and that they are unlikely to use the data in truly shitty ways (companies just arent getting away with that shit anymore), but it would still be a direct downgrade to mumble, which collects no data (except for setup details to help developers provide sane defaults and increase supported systems).
On a technical note, discord isnt working for my microphone on Linux, probably because its USB. This isn't a discord bug, but one with chrome I believe (as google hangouts do the same thing), but mumble literally had 0 problems at all.