The problem is that the extension experience can be really bad. There is no extension API; instead Extensions have (almost) full access to GNOME Shell's code.

This makes them incredibly powerful and flexible... but also fragile. Extensions can crash GNOME Shell/mutter. On Wayland that means your entire session goes down with GNOME Shell. Extensions can interfere with each other, and if you are an extension developer, you may need to update (or at least check) your extension every 6 months (GNOMEs release cycle).

Extension lives in the same memory space as the shell, so it’s up to the developer to restrict themselves to not touch internal API. Also, GNOME give you plenty of warning in the changelog (and the changes are usually small).