It's the wrong tool for him. OK. No problem. So is an 8x10". Doesn't make either bad.
Additionally, he's heavily invested in Nikon, and that is fine as well. ... sort of ;-)
The idea that there is a linear increase of camera quality all around does not apply. The 5Ds is striking example of a lateral off-shoot. The complaint that unless you use spot-on technique, you get poor IQ, is like complaining about the lack of ground clearance of a Ferrari to go off-roading. The 5DsR is a specialized tool, and unless you are willing to treat it as that, you will not be happy, regardless of who you are. MT is the poster boy for that. All the creative vision is for naught, unless you also use top technique with this body.
P.S. His comments on exposure like on slide film made me chuckle as someone growing up on slide film. The complaint about evaluative metering is equally funny. Center weighted and exposure compensation, or as ultimate approach, incident metering with external light meter (I use Sekonic 558L) are available. Why not complain about Program-auto not being spot on? Or AF functionality with Zeiss lenses? MF being hard, well, maybe use dedicated focusing screen.
Battery life is a bit on the low side. I've noticed that even with grip. But nothing unmanageable. Again, specialized tool, specialized technique, some trade-offs.