I’m leaning towards vendoring for all my new projects.

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.

> building your own UI library

It's one more thing to maintain, and it's also difficult to push back on things. If you use off the shelf components it's much easier to say to designers and managers that a UX pattern is not available or not valid. You can point to the mature well known community owned UI library you use and make it authoritative. It's harder to do it if you build your own, suddenly each designer and developer is throwing things in there, adding features etc. It's also difficult to agree on the structure, so the components are well thought out, flexible, but also not so flexible to lose semantics. It's not an easy job, do you use slots, composition, rendering callbacks, there are too many decisions and you spend time building the UI library instead of actually shipping features.