The one downside I've found (other than noise) running a super old supermicro at home is the power consumption is nowhere near as good as the modern hardware. But my server is _really_ old, some dual socket xeon thing I got super cheap.
Weird question, if you put an old, inefficient server somewhere sensible, could it reasonably pull double-duty as a heater? If you're pumping a kilowatt or two into it, that's enough to warm up a decent sized room!