Have to say, midi clock timing on my desktop system is actually pretty good.
I have Cubase 7.5 (PC) sending midi clock out of 3 RME Hammerfall Multiface units, all connected to the PCI-E bus.
I’ve never had great experiences with USB MIDI interfaces though. I suspect USB doesn’t really have the right protocol for this kind of real-time mechanism. Just my anecdotal experience though. 