I get the feeling you’ve never ran n>1 Xbox Ones connecting to Xbox Live at the same time. Without UPnP only one will be able to connect.

So is this issue mostly with consoles? I've always kept UPnP off and we do lots of gaming here without a problem, but pretty much all PC gaming.

PC also has problems. Truth be told it’s all about the kinds of games you play.

You can port forward of course, but you have to know which ports and obviously it only goes to one static IP

I can't say for sure, but I have never ever seen a PC game using UPnP. That said, I have only ever seen it once with a console, a PS3 in this case.

And, don't quote me on this, but most PC games are not Peer-To-Peer. They often come with their own server software.

Yeah, which pretty much requires you to do manual port forwarding. And if you're behind a CGNAT, you can't host.

I have and I manually manage my firewall. I have never seen a game that only uses/allows one port so IMO it would only become a problem with something like 10+ consoles playing the same game at the same time and all of them being a host. If even then.

Do you mean TURN? STUN does not work over Symmetric NAT as the source port is unpredictable.