I'm enjoying KUP on mobile but I'm not using the experimental mobile UI for a couple of reasons. The thumbnail previews automatically render as portrait even if the image itself is not, leading to a lot of wasted space. In fact I'd love an ultra compact option for KUP to further condense things, as there's a lot of whitespace. The images/icons for going to the comment section and boosting are very nice, but they do take up more space than just the text. I used RiF for reddit partly because of how condensed and clean it looked even with thumbnails.
Also it would be lovely if you could add a quick reply button under the article text when you click on an article, as right now I need to scroll down through all the comments to get to the Add Comment box, and of course having infinite comment scroll on can make that a bit of a chore. I'd argue that's perhaps something to be addressed on the site overall, but since @ernest is working pretty hard on the back end, sometimes you need things like KUP to help out on the front end for accessibility and minor QOL issues.
Fwiw I am using Firefox with tampermonkey on a Galaxy S22 Ultra.
Frantically checking my email in hopes I'm finally in.