Back when the Hexar RF was still in production there was a big deal (scandal?) revolving around the M-Hexanon lenses not having exactly the same exit-pupil-to-image-plane standard as Leica lenses. E. Puts wrote quite a bit about it. It was controversial to be sure, one reason being the suggestion that film flatness/curvature/emulsion thickness played into the issue. Since the M8's image plane is not curved, and focus issues seem to be more critical, I would ask: do you use these lenses on an M8, or are you opining based on using them on film?
I use them on an M3, an M5 and the M8, I have not ever seen any focus shift on either of them. Used the 50mm f2.0 on Minolta CLE, no trouble either. I shoot the 50mm f1.2 on M5 and M8 currently (the M3 is sick, it needs to have a CLA) and it's doing fine on both. About focus issues being more critical, I do not agree with you, the 1.33x crop factor on the M8 slightly increases DOF, so focus is less critical, albeit marginally.
Erwin Puts is putting off anything non-Leica all the time, IMHO. Personally I put no faith in his reviews. According to fellow RFF-er Maddoc, Tommy Oshima shoots a Noctilux on the Hexar RF, if there were any truth in the story he'd have trouble with that combo. I put no faith in the film curvature story either, film does not curve differently when a Leica lens is mounted.
I can't speak for the esthetic taste of others, but I own that lens as well as a 4th-gen 28 Elmarit and I have no idea what anyone could be seeing as "harshness" with the Ultron. I would be the first to admit it is no 28 Summicron @ f/1.9-2 in terms of corner sharpness or contrast (not that it matters to me for the type of shots I make with it at that aperture), but otherwise it's a splendid lens in every way I can think of.
Like said, I have no hands on experience with the lens. Not trying to put the Ultron down here, just pointing out the existence of a thread were several people stated disappointment with the lens. The OP might wanna get in touch with them and ask for shots while orientating.
I don't know who underrated it. Sean Reid compares it favorably to the 75 Summarit, and it holds its own to the 75 Summicron and Summilux at least by f/4-5.6. I've never read a bad review of it. I think that the 75mm focal length generates more interest now with the M8, because the crop factor makes it the closest effective 1.33x length to the ever-popular 90 on full-frame.
I say, it's underrated here on the forum. I see a lot more shots with the Summilux 75mm than with the Heliar 75mm on this forum. Nobody ever mentions it in threads.
Anyone wanna throw in some Heliar 75mm shots, for good measure?
😉