If you pair a 5DS R with a Zeiss Otus... and you pair a 645Z with any of its typical, similar lenses...
Larger sensor and much more expensive body (Pentax set), vs more expensive lens. (Canon set). Both camera bodies offer a similar sensor resolution (of around 50 megapixels).
I'm curious how the sharpness would compare.
Note that Pentax's lenses are significantly cheaper (otus is ~$4k, pentax is ~1.5k, as at B&H).
I can give you generic principles since you don't mention the lens of the Pentax.
Bigger lenses with corresponding longer focal (for a given field of view) are easier to manufacture, they may also have less distorsions an aberrations.
Bigger sensor elements on the sensor chip (5.3 micron for Pentax, 4.1 micron for Canon) mean simpler microlenses, and also less blur: consider that the "diffraction spot" (related to the smallest details that you can actually resolve) is about as big as the aperture value. If you have 4 microns, you need to shoot with f/3.5 or lower to record single-pixel fine details (extremely contrasty details like white on black are easier to resolve, I don't mean those). On the Pentax you can shoot almost one stop higher and retain maximum details, meaning that you can use your lens with an aperture that more easily produce sharper images (ok, the Otus is always sharp, that is an exception).
And keep in mind that at high ISO the Canon should be better than the Pentax, medium format has always had poor noise values at more than 800 ISO.
(1) Irrespective of the lenses used (Zeiss Otus for example), a Nikon 810 or Canon 5ds can't produce detail that the sensor can't pickup to begin with. A small sensor can't resolve the detail that a larger sensor can even if you put an average kit lens on a 300mp camera. You'll get more detail with the kit lens and larger sensor compared to a great lens on a smaller sensor.
(2) I can use a run-of-the-mill 55mm f/2.8 lens on the 645Z and get better detail than using an Otus lens on my Canon or Nikon. Who cares what's sharper out-of-camera? The fact of the matter is that once the files are processed and sharpened, the detail is ultimately better from the larger CMOS sensor practically every time. I wouldn't go gaga over using an Otus lens on a small sensor camera... it's like a microbiologist using expensive highest quality glass slides to put samples on, when s/he's using a $50 microscope. It doesn't make sense.
(3) Many aren't used to shooting medium format (MF) and still think that MF are horrible when raising the iso. This hasn't been true for years now. Every since MF started using CMOS sensors, MF has been the highest quality high iso shooters on the market, and it is still that way today. You cannot get the same high iso quality from a Canon, Nikon or even Sony when compared to a CMOS equipped MF Pentax 645Z.
Most people don't have both 645Z and 5DS with Otus series lenses and I would suspect that lots of comments you read on this topic will be opinion based and biased towards what people own or like.
I think that closest to fair and balanced comparison is to find samples from both cameras shot in dpreview.com reviews and comparing them on your own. if you are serious about the problem, I suggest that you select some pictures and print them in sizes that interest you. I suspect you won't find many samples shot with Otus, if any, but reasonably stopped down high end Canon lenses won't be much different in terms of resolution.
