They’re pretty lacking. <select> is a great example: no searching/filtering, no ability to have styled/multiline text in <option>s, nor anything that’s not plain text (so no SVG iconography for example).

People constantly have to reinvent the wheel to offer fairly basic functionality, so it’s good that there are libraries to handle the behaviour and accessibility side well while supporting whatever visual styles you want to make.

Some common, but complex, UI components are hard to get right, especially when it comes to accessibility. Combo boxes in particular come to mind.

There’s a lot of UI elements that are not in the set of html elements. Sometimes you need different behaviours.

I think it’s still best to build on the native elements first though.