The UHD is a way of coding the video burst, where the overall imagine is divided into, approx., 4000 pixel per line. There are several different coding standards referring generically to UHD, but... this is what they call it.

Moreover the quality of the backlight of the screen you’re using is really fundamental to assure good contrast, luminosity, deepness of the dark level and a general good impression in viewing.

Because the pixel dimension, that’s the balance between the dimension of the screen divided for the number of pixels itself (4k for UHD or 1920 for horizontal line of FHD), might be so small, as to be insignificant for our eyes perception.

The real variables involved into a generic “Yes I like how it looks” are quite a few, and some of them are subjective. But generally we can say that in the “personal device” area between 7”and 12-3”, a UHD and an FHD display could present the same “ viewing experience”, and because UHD is much more expensive in HW, and heavy in SW, it is really useless….for these small displays.

Going ahead to the next advertising level, between 15” and 43”, the argument is very questionable, because as I said above, there are a lot of variables involved. But let me tell you, from my experience, that a good quality FHD@1080p, could appear better than a low quality low cost UHD.

Going to the digital signage size of LCD, up to 83” range, there’s no argument to be made. UHD appears really much better especially if you view from nearby.