How does shiny ABS feel compared to POM?
Not the same at all. Shiny ABS has a disgusting greasy feel that comes from some part of the brain. You can wash and scrub your shiny ABS keycaps with grease-removing soap and water, and they will always feel greasy to your brain. It has something to do with the nerve endings in the fingers and the sensation they send to the brain.
POM feel totally different. They don't feel greasy at all. I guess "wet" or "slick" are appropriate adjectives for POM, but they are lacking and don't fully convey the feeling of POM. I would also add "fast", "cool to the touch", "smooth", "resistant" and "resilient", but even those words do not adequately describe the sensation. There are some things that cannot be expressed in words, or at least not in English words. We really need to invent new words to describe the way keycaps feel on the fingertips, just like the Eskimo's have 180 words to describe snow, we need new words for the feel of keycaps. (And like the Japanese have the word "umami" to describe a certain savory taste in food, that is untranslatable in English, since we have no word for it, although we recognize the taste in our mouths.)
Also, POM keycaps require laser-etched lettering, or laser-etched in-filled lettering, which can feel like a raised bump where the letter is. Some find this annoying, but I do not. In all I would say POM is the great under-appreciated keycap material. It is not as useful as PBT, but it's infinitely better than ABS.