Wow, you have been making these for quite a while. IMO, kinda monotonous and eventually loses its originality, but whatever works for you, I guess.
My own opinion may stand out very sharply compared to your watchers among other people, even my own from the last time I sent a critique for one of these.. Of course, the level of detail will certainly impress a number of people. My personal reaction looking at these wallpapers for the first time in months? They're not very convincing for the level of realism that was probably intended.
I notice that your rocky hill is very lumpy. Looking at whatever photo referencez of rocky cliffs I have found, I think I know why I seem to be bothered by the lumpy shape of the rocks. That's because real rock formations aren't gonna be lumpy at all. I believe that any protrusions of a rock will be eroded away due to having more surface area. Rocks also don't apparently crack in curves as suggested by whatever photos I've seen around; when a rock is cracked, they apparently make straight cracks. If we want convincing realism, we will need references to know what we're exactly modelling.
Either that or I'm looking at the wrong references
The rocks also appear to be a solid color. Real rocks are a mixture of many different minerals so they should be much more colorful than what I see here. I am no 3D modeller so I am unsure now to go along adding color to make it look like its made of minerals.
We also ought to pay closer attention to light sources. Your main light source is Twilight's horn making some magic. If you look closely at which planes are lit, the rocks suggest a frontal light source rather than one from Twilight... albiet for some reason only lighting up areas near Twilight. Coincidentally, I hear that frontal light sources are default for 3d modelling... hmm. I think we ought to be more careful here.
Then, there are also fog effects that personally, I am not seeing much artistic merit. What is it adding to the piece, exactly? It doesn't add to the setting like the rocks and the sky. It doesn't make sense to think of it as mist as such a humid environment should produce a bit of vegetation on the rocks even if it's just moss. It could be adding detail, but as far as I've understand silly art stuff now, details aren't always good or necessary.
Another thing I start noticing is that the poses for your vectors are made for poses that assumes a flat ground. Kinda strange given the uneven rocks that is sure to cause major headaches to equines that belong on the plains.
Many people do love these wallpapers, but they just don't seem convincingly real for the amount of detail put into them.
I remember you mentioning that you do favor these sorts of special effects considering your educational direction, but ponies is a pretty difficult subject to use for this sort of art style. You have minimalistic vectors, and special effects that tries to satisfy horror vacui. I don't know what others think, but that combination feels especially wierd now that I have a much better sense of distunguishing art styles.
Sorry if its one of the few critical opinions you ever get, but its probably a very useful opinion if you ever wonder why you aren't getting one more watcher than what you have as of now