@Brimstone Discord does have legitimate privacy concerns but, instead, I want to draw attention to more practical concerns.
Firstly it is a worse performer than Mumble. It has higher latency, the overlay feature does not work with TF2 and consumes more system resources. If TF2P wants to be hailed as the ultimate competitive TF2 platform it's strange that that the mandatory VoIP program isn't the one that provides the most instantaneous communication. The lack of overlay creates the problem of voice comms becoming disorientating particularly for new players who may not recognize their teammates voices which can lead to frustration. "Uber me!" "Okay, here we go!" "I'm dead, why didn't you uber me!?" Furthermore, the increased system requirements alienates a small, but valued, playerbase who rely on every iota of their hardware to run a stable TF2 client which we all know to be increasingly demanding. see
Relying on a third party service for the livelihood of your own is not logistically sound. It may be decent now but as Discord matures there's no telling how reliable the service will be. A change in licencing, function, or policy you may not expect can easily cause grief for TF2PL later down the line. On the other hand Mumble is not a service but a tool. It's decentralized -- anyone can fork the program and anyone can host a server. Reliability is completely in the hands of the user.
Discord does many things better but this is not one of them.