generally I like how gtk4 has improved a lot from gtk3 (and in some ways also improved from gtk2) but man is its smooth scrolling *unpleasant* to use. In some programs like kgx/vte, at least.
In nautilus it feels okay (which is fortunately my main gtk4 program), but in gtk4 libvte terminals or the new gtk4 PDF reader it feels like it's completely stripped of any control or precision – like as if it's been deliberately slowed down so that you'd be required to "appreciate" the new inertial scrolling feature. Want to scroll more than 3, 4 lines? Have to slam the full height of the touchpad a few times, then let go and let the scroll physics take you somewhere nearby.
(unless it's inside Vim, then it sends enough mouse events to scroll two whole pages from a little finger twitch.)