I also know input methods work great on Ubuntu (Gnome Wayland) because Gnome has first-class support for ibus.

But the article is specifically about incompatibility of input methods among different DEs: Gnome, KDE, wlroots-based environments such as Sway and Hyprland.

After many years, ibus still doesn’t work with wlroots, because ibus supports input method protocol v1 and wlroots supports v2: https://github.com/ibus/ibus/issues/2182

The list of recommended IMEs on Sway are all WIP: https://github.com/swaywm/sway/wiki/Useful-add-ons-for-sway#...

There is even v3. Kitty refused to implement v3 because v3 appeared even more unstable: https://github.com/kovidgoyal/kitty/issues/2814#issuecomment...

The wayland input method protocol is stuck at v1, and DEs/applications disagree about which versions to support. That’s why it’s problematic.