Leica R lenses are probably easiest to use on the Sony A7 series these days, or on the new Leica SL, with an adapter. Leica R lenses are cumbersome on Nikon, been there done that. You learn that auto aperture actuation is really a nice invention. Nikon and Zeiss ZF are so much easier to use.
What I really like about the Leica is the small sizes of their lenses.
For example, the Leica Apo-Summicron-M 1:2/50mm ASPH is on a par with the Zeiss Apo Distagon T* 1.4/55 Otus optically ... but the Leica weighs only 300g whereas the Zeiss weighs almost 1000g.
However, the Leica (while a third the weight) is
twice the price of the Otus.
The Leica is not any better optically than the Zeiss; it's just twice the price.
However, the tiny size of the Leica (while sporting that kind of quality) is not only more convenient, it's less obvious and obtrusive.