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
I'm not disagreeing with any of the technical points in the article. I'm just saying that my experience with Wayland is pretty good. Someone (GNOME? Ubuntu? thousands of generous developers?) is doing all the work to make it work pretty well for me.