With Cosmic they very much want customization. That was one of their primary goals. The very reason they build Cosmic in the first place was because Gnome made it much harder to costumize and broke all their existing plugins. They are talking about Cosmic being able to serve as a great base for costume experiences.

I am not sure if they have reach that goal, but that is their goal.

With Gnome, they are very willing to break all plugins at all times. They simply do not care for stability of plugins, its a nonissue for them.

In Gnome plugins are JS application all running in the same namespace, a single crash of those can take down everything. In Cosmic these are Rust application running in different namespaces.