I have both an OG model M and a unicomp model M mini. They both feel great and seem to have zero issues. My first model M was a 1987 and I used a string of pigtail adapters to get it from AT to USB and while it didn't start working until my OS started booting, it was my favorite of all time. It got lost after we went work from home during COVID and they had a moving company pack our desks up. There is not a day I don't miss that keyboard.
With all that said, I don't have any experience with the rp2040 controlled unicomps. If you can get an actual model M on the cheap, I say go with that. If not, unicomps are a fine product in my experience.
The only thing I can tell you is that if you get a keyboard with a bad membrane, it CAN be fixed but it's a huge pain in the ass since you need to replace all the little plastic tabs on the chassis with tiny bolts.