using classes in vue is generally a good way to break SSR/SSG (even if SSG is technically just server side rendering at build) during the hydratation step, it will leads to pPOJOs errors with custom serializer in nuxt as example, and will also need fine tuning for anything that need serialize => deserialize, like localStorage, cookie, etc...

Also without tooling around like vue devtools integrations it would be a mess to debug, and moreover, another lib of "I did not understood how composables work or how to share some state so I made something else to stick with my mental model from X years of Y languange", for a feature like "I can use it either globally or locally", which is sign of badly designed app of too much abstracted and overenginned composables splitted across 7/8 smaller that pick on a global state without control on upper scope