Couldn't you still fry a PSU if the cable assumed multiple common pins to the PSU, but the PSU had different voltages across those connectors?

At least for the layouts listed here: https://pc-mods.com/blogs/psu-pinout-repository

The motherboard cables do all kinds of different scary crossovers so never separate them, though you'll probably never have an "extra" motherboard cable in the first place.

None of the other cables bridge pins on the PSU side, they "just" scramble the pins they're attached to at the other end.