Grabbing an off-the-shelf UI library is easy in the short term, but it’s usually overcomplicated, implements things I won’t ever need, is hard to tweak if/when you want to distinguish your app from the thousand others using the same library, and when you do decide to upgrade it, all your tweaks break in subtle ways.

What I think would be the best approach is building your own UI library. You own it, you get to reuse it across different projects and maintain the same visual style (if desired), and you add features when you need them.

Vendoring your components gives you the best of both worlds. You get a full component library but retain the ability to modify them as you want.

Your AI agent claim doesn't make any sense either. When upgrading normally your component just gets rewritten on disk. When switching from radix to base ui, a more comprehensive approach is needed.