On a technical/complexity level, IP NAT is much worse because you need to hold much more state, i.e. you need the UDP/TCP flow information to rewrite correctly.

However, MAC NAT is the pinnacle of stupidity for an entirely different reason: there should be no need for it. MAC addresses only have local significance, and while there are some long-term concerns about them running out, there is absolutely enough of them right now. There should simply be no need to do MAC NAT, if only it wasn't for the shortsighted 802.11 design decision to go with 3 addresses in the header.

FWIW, MAC NAT is almost the same thing as a router with ARP/ND proxying turned on, though possibly implemented on a different level. This is the technical reason it's an extremely stupid hack: proxy ND/ARP provides pretty much the same thing, but in a much cleaner way. (The difference is that with proxy ND/ARP, the "router-ish-bridge" assumes ownership of the lower-layer exchanges, i.e. ARP & ND, and just does normal routing with that. MAC NAT, meanwhile, tries to be clever and just forward ARP and ND. Reasons for doing that are ... extremely thin IMHO.)