There are certain kinds of fully custom keyboards, such as the one I use (Dactyl Manuform) where the keys do not sit on a single flat plane. Such a build would require a flexible PCB. This is doable for the advanced-amateur EE but the geometry is fiddly, at best, and doesn't allow for easy experimentation on key placement (spacing, offset, radius, etc, the bigger problem for me).

I've found the happy medium with single-switch PCBs ("Amoeba" is a popular one), which are fast and easy to wire up. Most notably you don't have to do the painstaking row wire stripping -- just use a plain wire from one PCB to the next in the key matrix (or an insulated wire if you have to traverse other components, which still only requires two stripped ends).

I have a few pictures floating around from my build(s) if that interests anyone.