If Cloudflare have problems, so does a large part of WWW.

Not Teamspeak (or Mumble) servers.

Much more susceptible to DDoS than Cloudflare.

You can just chose another server. Can't do with Cloudflare (or Discord).

Discord "server" admins can set a different region for their server to work around some types of outages. I've had to do this once.

I am not talking about public servers. If your team/clan's voice server is being DDoSed. Bonus points if a member hosts the Mumble or TS server.

There is not much difference. If your team's voice server is getting ddosed (for whatever reason), there are lots of public servers that allow channel creation.

Yeah, and there's Discord, which also has group chat with history, and a decent interface.

What Team Speak, Mumble, and especially Ventrilo have going for it is that they are more lightweight. However, Ventrilo has a strange latency which cannot be solved (except by moving on). Team Speak, Mumble, and Discord don't suffer from this. Also, Ventrilo cannot be self-hosted (while Team Speak and Mumble can).