UDP is actually more expensive to NAT than TCP is. The reason is UDP fragmentation, which is my vote for the worst, and least forgivable, design error of TCP/IP.
Instead of putting the fragmentation in L4 (like QUIC now does) and including a UDP header on every fragmented packet in a datagram, UDP only includes the header on the first packet. With fragmentation happening; firewalls, NATs, and end-hosts have to buffer and coalesce IP packets based on IP IDs, before the destination can be identified. It's a real nuisance. A lot of CGNAT "stateless" implementations can't handle this and you get very hard to debug issues when there are fragmentation and MTU mismatches.
