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.
I feel like a big problem here is that the Getting Started starts with a bunch of code that just... outputs a bare button. Starting with something like a combobox would show much more value.
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.