> UDP-based protocols are well suited for P2P, since hole punching is straightforward if you have predictable port mapping, you cannot disallow it.

"Cannot" is a strong word:

> UDP hole punching will not work with symmetric NAT devices (also known as bi-directional NAT) which tend to be found in large corporate networks. In symmetric NAT, the NAT's mapping associated with the connection to the known STUN server is restricted to receiving data from the known server, and therefore the NAT mapping the known server sees is not useful information to the endpoint.

* https://en.wikipedia.org/wiki/UDP_hole_punching#Overview

I've also heard lots of people complain about how they're stuck behind CG-NAT and various P2P things do not work.