Accessibility will rely on correct use of landmarks [1] and aria attributes, as well as real links (like the article mentions). Custom elements can do that just fine, by either decorating them with aria attributes manually and placing them in or around landmark elements, or by having the JS side of the custom element take care of that automatically. The JS class can generate any necessary markup inside of the element to help make it accessible, just like a component built with a JS framework would, and by using shadow dom and slots this markup can wrap around the child elements (though I think it's best to try to avoid shadow dom, as it is quite a cumbersome API to deal with). UX is a bit of a similar story, any kind of UX can be achieved once you register a JS class for the custom element.

[1] https://developer.mozilla.org/en-US/blog/aria-accessibility-...