I did a lot of legacy dslr lens testing with the a7r, and at 24mm and wider, the old lens designs were really weak, compared to what's available today... forget the old ultra-wides, especially in m-mount.
Just to set the record straight, there are several UWA lenses in M mount which work quite well with Sony sensors and are good (Leica WATE, to mention one).
the wide-end pics that I've seen out of that lens were not impressive, in the technical sense... modern ultrawides on the a7r blow it away, so it is not just a sony sensor issue.
The WATE is *very* sensitive to adapter length, being a UWA with floating elements. Are you sure you've seen some shots taken with an adapter of the right length?
have you ever seen pics from a modern ultrawide lens? here, i'll post up a sigma 14/1.8 test shot, ooc jpeg, taken wide open.
I sent that lens back because of the big mid-field soft spot on one side, but the potential should be clear, look at the sides and the top-left corner... wate is an ugly joke compared to that, it doesn't matter what camera body it's on, or what adapter is used.
While it's true that *most* rangefinder wides and ultrawides don't do well with Sony sensors, a blanket statement as above is inaccurate.
the only thing that is inaccurate here is you incorrectly paraphrasing what other people say.
If anyone is paraphrasing, it's you. I base what I say mainly on my own experiences. You've *seen* some pictures taken with a WATE, I've been *using* one for years and have compared it many times to more modern UWAs.
see below...
legacy ultrawides bring the suck, mostly because the lenses are weak... legacy wide lenses like 24mm are better than that, they are usable when stopped down a bit.
Very cohrently and scientifically expressed...
the correct spelling is "c-o-h-e-r-e-n-t-l-y" ;-)