I've used all the digital M's for landscape work over the past seven years. Provided the weather wasn't too extreme and you had enough batteries, the earlier digital M's made excellent choices for this owing to the relatively light weight of a Leica kit (assuming you need to hike any distance to the place you want to shoot) and their lens' superb optics. For me, one of the most attractive aspects of using the Leica system for landscape work is the DOF scales on the lenses. For landscapes I am generally looking to maximize depth of field while limiting diffraction and the Leica lenses have the great advantage of well-spaced and detailed DoF scales on the lens barrels. You can focus first on the nearest plane that needs to be in focus and then the farthest, note the settings, and it is then easy to estimate required F stop and hyper focal distance. (In contrast, Zeiss manual focus lenses, even the largest, have DOF scales that are too compressed to be of much use and few autofocus lenses from any manufacturer have any useful scales at all.)
The new M addresses the weather sealing and battery issues of its predecessors and overall is a more refined product. The optional electronic viewfinder is not great by current standards, but still helps a lot in many circumstances.
The Sony a7A, which I am now testing, has some important advantages over the M body for landscape work, including absolute resolution, superior low light capability, a far better EVF and, most importantly, the ability to move the focus point to almost any part of the live view screen and to magnify it to check focus. If the a7R sensor had been designed to work well with rangefinder glass, I would not see a need for an M body. Unfortunately, most Leica glass I have tested on the a7R is compromised to some degree or other in the corners. The WATE and some of the >50mm lenses, stopped down to at least F8, are better but even with those lenses I think images from the M are more convincing.
The Sony could be a competitive option with native glass once Sony fills out its lens line. The 55mm and 35mm primes are excellent; the 24-70 is also quite good between 30mm and 60mm. These lenses are all light weight by full frame DSLR -- and with the primes, even by Leica --standards and are weather resistant.
Meanwhile, the Sony can be a serious alternative if you are also prepared to use the best Nikon, Canon and DSLR Zeiss lenses along with those Sony FE lenses available at this point. For example, you could carry a kit including the Canon T/S E 24mm, one of the very best 24mm lenses, the Sony FE 55mm and the Zeiss 100mm Makro Planar in Nikon mount (as I did on a recent hike) and get image quality at least comparable to that of the Leica kit I would normally carry (24mm Elmar, 50mm Summilux and 90mm Elmarit) with the T/S option to boot. The downside is that this kit, while lighter than a full frame DSLR rig, is still a good deal heavier than its Leica alternative; the Leica kit still wins if much hiking is required.
Those are just my own preferences, of course. We are lucky these days to have so many options.