I can't make any claims to contrary about other providers, but I know at the very least that at one point not in too distant past the primary systems used for Rackspace Cloud hypervisors were Dell R720 rackmount servers. Maybe not the most amazing hardware, but considering how common they are you can hardly refuse to say they're "server-grade". The newer OpenCompute stuff is also clearly well-made hardware.

Everything I've read implies that cheap commodity servers like Open Compute are just as reliable as name brand Intel servers (not surprising considering that they're made from the same parts), and ~95% of the market appears to be satisfied with that level of reliability.