No. Most NATs handle UDP sessions trivially as long as one end of the connection is not behind a NAT itself. Tricks like UDP hole punching are necessary only between two endpoints that are both behind NATs.

And assuming the protocol can cope with changing source ports. I’ve seem some udp protocols that only work if the source port is not translated.

Most NAT still allows UDP hole-punching (STUN/TURN/ICE) unless specifically blocked (which a depressing number still do).

Yes. NAT, firewalls with admins that don't know what QUIC is, various corporate MITM boxes, etc.