Until very recently you couldn't really use a Wacom tablet on a Linux desktop that was using Wayland instead of Xorg. There is a major difference between X and Wayland in terms of tablets:
Previously we have been able to simply have the pencil move the main cursor whenever it gets close to the tablet. This worked fine but the Wayland way is actually better and allows for other functionality, unfortunately it means applications needed to be updated to be multiple cursor aware.
Until recently, xwayland (the component that translates Wayland events to X events for older applications) wasn't able to transform tablet events at all, since the Wayland tablet specification wasn't finished.
Things have improved a lot and now a default installation of Gnome Shell (in this case in Ubuntu) mostly works, but in my case at least it still needed a simple tweak that is not particularly easy to guess. Documenting it here for posterity:
gsettings set org.gnome.settings-daemon.plugins.cursor active false
Explanation: There is a running plugin in Gnome Shell sessions called "the cursor plugin" that apparently tries to help by hiding inactive cursors. Whatever it does, it breaks the Wacom cursor, but since it's not an essential component we can just disable it (set it to false) which is what the above command does. Set it to
true to revert.