You've had a few replies so I guess mine will be lost to the aether.
NAT vs Direct addressing is an interesting topic, because we've gotten so used to working around the issues inherent in NAT that we take them as a sort of given. I'll lay them out here:
1) The actual NAT state table in your router is much slower than a simple bit-map firewall lookup. This will show up as a bit of latency on every new connection.
2) The state table can get full. When that happens some connection needs to be evicted. For web technologies this wont look too bad.. Maybe a websocket connection gets closed and re-connects in the background. But if you're streaming something over raw TCP then that's annoying. Basically it makes your internet connection just that little less stable.
3) uPnP exists to try to mitigate the p2p issues with NAT; but does a poor job. -- Take for instance, a video game with VOIP, consoles are notorious for this; centralising and muxing everyones audio is expensive, so it's more useful to help people build peer meshes. So "NAT PUNCHING" is the normal way to go, but of course that doesn't always work, so you have weird tutorials on "how to port forward" when in reality this shouldn't be needed, a stateful firewall would be enough if not for NAT. Some guides even suggest putting your devices in the DMZ with direct port forwards on every port from the internet[!!]
https://www.denofgeek.com/games/how-to-change-nat-type-on-ps...