How is Cosina's quality control and uniformity

[Einst_Stein]

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While looking for a Zeiss manual focus lens, I encountered a review on Zeiss Distangon 15mm f2.8, which said the new version of the same design of a lens (Milvus 15mm f2.8) had bad quality control. The author tested four samples from different sources and found none were up to the expectation. All four are far worse than the classical version.

The Milvus version 15mm f2.8 has the same optical design as the classical version, except the improvements on the lens glass coating, added weather seal, and switchable aperture ring clicks. So intuitively Milvus should be equal or better than the classical version. If it is worse, far worse, than it is likely problems in quality control or manuafacturing management. 

This is the only review I've seen reporting the bad quality issue. All other reviews I've seen have only good words about the Milvus version. All other reviews I've seen might be only repeating Zeiss' ads, with some sample pictures, of course. I am not sure how and who to trust.

The weather seal and the non-click aperture ring is attractive, but the sample quality variation is alarming too. 

How would you go further on the reported quality issue?

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[Pieter12]

All the Cosina lenses I have are excellent: Zeiss ZM and Contax, as well as Voigtlander. The Zeiss ZM lenses (50 & 35) are the equal to anything produced by Leica.