Cover/Flip switch and Breeze work, but most others don't. They sometimes activate properly, but most times nothing shows and the apps are simply switched by alt-tabbing.
which "other"? KWin does not ship any tabbox themes at all.
Perhaps I filed this in the wrong spot, but under kcm's task switcher settings under Visualization, there is a large list of themes including cover switch, flip switch, breeze, breeze dark, informative, text only, etc. In a wayland session, nearly none of these themes work except the ones I mentioned earlier.
Those switchers are technically not part of KWin, they are just addons distributed with kdeplasma-addons. I can confirm that they do not work at all.
Also affecting me with Plasma 5.9.4 and the "Large Icons" Visualisation.
This should be fixed in latest KWin master. Relevant commit: https://cgit.kde.org/kwin.git/commit/?id=ac08c5ac9d4532b2506c76e8b69249b32d365aa3