Confusion regarding Turn servers

[linkstandalone] If there's something I never quite understood, it's TURN/STUN servers. That being the server-side software needed to setup voice chat for other software like Jitsi and, of course, Matrix.
One especially large annoyance is that even after forwarding every port imaginable, the server still won't work under an NAT. This is such an issue that the developers of Matrix's flagship server software, Synapse, even point it out in their page on turn server how-tos.
Just a few notes.