Why is RTSP over UDP expected to fail when using a NAT/Firewall?

[charlieblevins]

Question

I see this in the README:

UDP: the most performant, but doesn't work when there's a NAT/firewall between server and clients. It doesn't support encryption.

It seems to have been added in this commit.

I have my suspicions, but I am really interested to know how this was determined. Why is this case expected to fail?