Haven't built one, but a couple of thoughts come to mind:
1. The LCD version of this (and maybe the solenoid/piezo?) seems to require destructive disassembly. It's the owner's board, but M122's are desirable to buckling spring fans, and it would be a shame to gut a working one. There are other ways to
get or make a Cherry MX battleship.
2. The PCB could benefit from some relief cuts. This is a lot of tension, and while using the thin 1mm as the designer says should help, a lot of the writeup seems to focus on managing that tension during soldering and assembly.
3. This is a LOT of hotswap soldering on pads to do by hand. I would not enjoy the work, personally. I think the whole thing would benefit from a reflow setup maybe?
4. Now all that said, this is really intriguing and I love the idea of solenoid keyboards.