I've been working on connecting two devices that are each behind a symmetric NAT and managed to make it work quite consistently. However, it takes quite some time for the connection to happen (sometimes more than a minute) and it will not work on certain types of symmetric NATs. I believe this is why STUN doesn't support symmetric NATs and that TURN has to be used.
But, when I was doing some tests I first started by trying to connect a device behind a symmetric NAT to a device behind a port-restricted cone NAT and managed to make it work very consistently in a short amount of time and on multiple types of NAT. So I'm wondering, do modern implementations of STUN support this feature? If not, why?