Supermicro boxes are generally:
b) rock solid reliability. I've only had one or two server failures in about 10 years where I've had to invoke RMA or warranty.
c) free IPMI with all features included, eg KVM and remote media
d) easily available with practically any combination of drives, backplanes, PSUs, and other accessories. I've got a couple of 1U boxes that even have 3 PCIe x16 slots available. Getting even vaguely custom builds out of HP has been way harder for us as an SMB. With SM vendors, no problem.
e) lots of warranty options
f) OEM front panel/bezel service available.
I think these are the reason that HCIA vendors' products seem to be largely based on OEM's SM boxes.
I am not affiliated with Supermicro in any way, just a very happy customer.