The power supply incompatibility came to fruition a long time ago. Buying used Supermicro ATX motherboards to build into servers stopped being a thing for me about 1 decade ago. But used servers and desktops, even with their non-standard parts, have continued to deliver high value for me even today.

When boxes get decommissioned it's generally the entire thing. So you can pick up used power supplies as well. Or just buy new because even if it isn't ATX it's still a widely produced item that's used across multiple product lines.

The troublesome hardware is the stuff with custom backplanes and multiple daughterboards each hosting a node. Also AMD CPUs that lock themselves to a single motherboard.