[Saris]

I wonder why NAT hole punching techniques never took off much with P2P, it seems like with enough work getting around NAT most of the time would be possible.

[qalmakka]

Good, reliable ways to do hole punching were not available until relatively late (STUN, ICE) and were somewhat finicky for a long time, especially on crappy routers.

Also, multiple levels of nat are unfortunately common and make STUN unreliable

[wickedsickeune]

You can't hole punch on mobile. Most mobile networks use symmetric NATs unfortunately.