Oh I can do backend work, but I'll easily introduce performance issues, log too much or too little, get lost as soon as I open one of our observability dashboards, or design an inefficient database migration.

Likewise, I've seen more backend-focused people introduce horrible accessibility issues, completely misunderstand the client-side lifecycle, or produce a giant mess of intertwined global state.

It's useful to be able to wield all the tools to some extent, but I've found true full-stack to be a mess. (Where "true" already means "works on web APIs and in-browser code", i.e. still ignores large parts of the stack.)