Being objective, for me is really the M lenses "despite" the M cameras... It is just physics or more accurately, optomechanics. If we follow history, the best lenses with less distortions and aberrations are the ones based on the symmetrical or pseudosymmetrical concept. These optical developments have a "slight" inconvenience and is the fact that the distance from the optical center to the image plane is approximately the same as the focal length of the lens.
Back in 1950, Pierre Angenieux and Harry Zoellner at Carl Zeiss Jena applied at the same time for their "Retrofocus" and "Flektogon" design. It is a inverted telelens that permits you to move the lens last element away from the focal or image plane. This will leave space for the mirror in the reflex camera and for the turrets in the cinema ones.
But increasing this "backfocus" distance (distance of the last element to the image plane) does not come "free of charge", we pay taxes in the form of increased distortions and aberrations that will never be reduced as in the symmetrical designs, or if these are reduce close to symmetrical values, the lens will grow phenomenally in size and weight (Otus?).
All this is valid basically for standard lens and down, I mean focal length <50mm... this is the main reason why there is no Biogon for reflex and basically we only find Distagons, derived from “distance” and the Greek word "gonia" for “angle”. Thus a Distagon is a wide-angle lens with a large distance to the image, and this is not the only "large" characteristic, because we all have clear that in size and weight these are light years away from the Biogons (ZM).
Even more, due to digital sensors characteristics (but this is another story) and exposure meetering, Zeiss increased in 3 mm the backfocus distance of the ZM family versus the G family (for the Contax G) and we all saw how the ZMs grew in weight and size away from the fabulous Gs...and also their distortions. If we check the back focal length of the Biogon 4.5/21, this was only 9 mm, in the Biogon 21 G, it increased to 12 mm. and in all ZM series lenses, the shortest back focal length is 15 mm.
Well, again, I think that this is already too much writing, sorry... hope this have been helpful and shows why I am in the M domain... ;-)
...and shows probably the brightest star in the Leica M constellation...