I use my R almost exclusively for landscapes and, in general, it is excellent at that task.
I'm on record here as someone who dislikes the EOS R quite strongly. It has many faults, some of them very serious, ranging from horrible user interface design and ergonomics, through lousy battery life, to substandard viewfinder and unreliable, imprecise focus system. My R doesn't get to do anything focus-challenging (i.e., birds and wildlife) because it cannot be relied on to grab a precise focus point and lock on to it, nor anything time-critical because it is slow and awkward and fiddly to operate. So for all those more challenging tasks, the R stays in the bag and I use the 5D IV, the 7D, or the 1D IV - all three are far superior.
Landscapes, however, are another matter. Time is generally not an issue (you can spend as long as it takes fiddling around with the R's cramped and clumsy settings); you seldom or never need to focus on a single precise point (like a bird's eye for example); and you often need to focus on an area which isn't under one of the small number of fixed AF points an SLR has - somewhere way down in the bottom left corner, maybe. With the R, you don't have to focus and recompose, you can just pick the AF area you want, no matter where in the composition that happens to be.
The R also delivers outstanding picture quality, with a richness that is ideally suited to landscape work. I know it is more-or-less the same sensor as the 5D IV, and in theory the results should be identical, but I can't escape the suspicion that that R delivers something even better than the already excellent 5D IV does. It's subtle, and maybe it's just my imagination, but I really like the way the R reproduces landscapes.
Awful controls aside (and we can live with them, I've used worse - though to be fair, not in the last 12 or 13 years since I sold the 400D), the biggest problem landscaping with the R is the EVF. It is very hard to adjust to not being able to see the subject properly. In good light it's not too much of an issue, but if you are out early or late, when the sun is low and there are interesting lighting effects going on, the R's EVF copes very badly. Strong sidelight is particularly difficult. Sometimes (in difficult light) I get tired of photographing blind and swap the R out for the 5D IV or even the 5D II for a little while until the sun gets higher.
At any time, but especially in awkward light, you have to learn to do all your composition with the naked eye, only looking through the viewfinder when you have decided exactly what will be in the frame and what won't be. (Contrast with an SLR, where you can look through the VF knowing only roughly what you want, and settle on your final composition in camera.)
I should also mention the dreadful electronic level. (What were they thinking?) Not only is it unreliable (much less accurate than the electronic levels in my various other Canon cameras, including even the tiny little Powershot G9X II), it also covers practically the whole screen, making it hard to see and frame your subject. Why? It was an insane design decision. I keep expecting them to fix in in a firmware revision but they have done nothing. Oh, and there is another gotcha - the electronic level doesn't work at all unless you first disable eye-controlled focus. (Watchout! They don't make that clear in the manual.)
I haven't mentioned using the rear screen. I practically never do that, so I'll leave comment on it to someone who has experience with it.
TLDR: The R is a deeply flawed product, however landscaping is one area where it performs very well. All things considered, it's the second-best landscaping camera I own, after the 5D IV. It is reasonable to expect from what we know already that the new R5 will be an outstanding landscape camera. I would give serious thought to waiting for that one.
