My FreeNAS has been running for years on an older ProLiant, off a USB stick, in an Ikea desk cupboard with poor ventilation (some gaps around the front door, and a 4 inch circle drilled in the back). The only failure so far was the old USB stick, once I reinstalled and put a new stick on I could see my ZFS data again.

I should probably update the software, but I already broke it once trying that, so for now it's sitting happy as a dumping ground for files.