Server PSUs lack the minor rails which are essential to any desktop PC to operate, and all desktop PSUs for almost a decade now have a strong +12V rail and as for efficiency, the desktop PSUs are on par with the server ones although they have DC-DC converters, which cost in efficiency, are much more silent and have way better ripple suppression.
I won't recommend to my readers getting a used server PSU (or a used PSU in general) which can blow sky high once loaded, since you simply don't know how heavily it was used before. On top of that, server PSUs are meant to operate 24/7, so don't expect to find a lightly used one, unless it had a problem from the start and it was sent for repair (and afterwards it is sold are refurbished).