The only complaint I've come across for the 5Dmk3 is relative to it's steep price, while complaints from owners? I don't recall anything specific, just maybe "oh man I wanted a 1Dx but could only afford this so my life stinks"
D800 complaints? Sure, plenty regarding the large MPs slowing things down, it's just unnecessary resolution for most. And of course the AF not being as stellar, and noise at high ISOs. I've seen one used Mk3 out of tons of electronic shops I've been checking out, and oh look there's a couple D800 s, and a D800E.
But, I find myself leaning toward Nikon for starting to beat Canon with video, but even if I would spend that much more for external recording drives for the uncompressed hdmi feed I'd rather wait and see what Canon will do in the near future.