Ignore the typical "use a D810/A7RII/5DIII/5Dsr" notes because they just don't understand ...
You can ignore the "5DIII/5Dsr" notes for sure.

Rest assured that the image quality you are getting from your D810 at ISO64 with the best lenses is a challenge for any digital back, including the very latest ones.
So the notes about the D810 are relevant if you don't know why you are buying your MFDB for.
This being said, it is true that the combination of larger sensor (at least from 36x48mm onwards IMHO), higher resolution (in some cases), different lenses,... can result in a different look. It is true also that the large OVF of these MF bodies results in a shooting pleasures miles ahead of that of an EVF body. It is true also that the slower paced shooting can result in a different vision and relationship to the scene and subject.
Now, be aware also that focusing can be a challenge, that handheld shooting can be tough, that higher ISOs are poor, that may important functional goodies such as Auto-ISO are typically very basic, that...
Cheers,
Bernard