Most reviews I have seen Leica beats Zeiss in the corners.
This is too general of a statement considering we have multiple sensors and film to compare it the lenses perform differently on all of them.
It seems to me like at one time Cosina wanted to have a premium line and an affordable line so they approached Zeiss. The ZM lenses were different but usually better than the CV line up until at one point Zeiss seems to have lost interest in the project. The ZM 35/1.4 being their last and best lens for the series. From the period where they were making lenses, optics like the 28, 25, 21(s), and 15mm were all world class. They were as good or better than anything Leica made with the same aperture
on film. The problem is they never updated the designs for digital so most of them are noticeably unsuitable for even the M digitals. I have shot the 28, 25s personally and they are incredibly sharp. The 28 worked well at least on the M240 I had. The contrast just sparkled.
Since ZM has lost interest you have begun to see CV lenses that are much more modernized to work well on multiple types of cameras. Their line up is massive at the moment and nobody can reasonably argue that of the lenses are substandard.
This guy does really good comparisons on his Leica digital:
https://jacktaka.com/reviews
I quibble with his conclusions on the 35/1.5 but his review led me to get the 50/1.5II, and if I wanted a 28 it would be a hard choice between the 28/2 Ultron and the 28/2.8 Biogon. I would never even consider a Leica optic. Not because they're not good, but because every option that costs a fraction of the price is SO good now.