I own a 50 Planar and 35 f2 Biogon and owned the retro asph Summicron. Also owned a 25 Biogon. Also for many years owned a V4 Summicron and have my 2nd V1 Summicron that I'm keeping along with my Zeiss lenses.
Several years ago when I bought my D800 Nikon I decided to buy a set of ZF Zeiss lenses. I had the 25 f2, 35 f2 and 100 f2 macro. Honestly I was quite disappointed in wide open performance. Everyone makes lenses that perform well at three stops down but wide open is what counts. The 25 and the 35 were soft in the outer zones and the chromatic aberrations were the worst I've ever seen at f2 and 2.8 with the 100. They were excessively heavy too. I kept than for almost a month and returned them for a refund to B&H.
In the 80's I owned 2 Rollei 3003's and A full set of Zeiss glass from 15 to 200. Other than the 25 and 50 I think all were German made Zeiss. Over the several years I owned them, and I used them very heavy, I found some had issues both optically and mechanically. My 35 1.4 wasn't very sharp until I stopped down two stops and then had the front element come lose. After getting fixed it still wasn't sharp. The 85 1.4 wasn't great at 1.4 either. In both cases my Nikkor 35 1.4 and 84 1.4 are as good or better. In the end I went back to all Nikkor glass and find it just as good as the Zeiss or better in some cases.
People equate weight with build quality. A heavy lens is not a better built lens. I've had more mechanical issues with Leica M lenses than with my Zeiss. I believe Zeiss elected to make the ZM lighter due to demand from customers and from experience they're constructed just as well. Think about all the complaining you hear about weight from Leica enthusiasts.