What makes custom elements good is inrerop. I can use them in react, Vue, angular, svelte, solid, or just plain ol' server rendered html. I can switch out the internals entirely for a WC lib, or not, with nobody knowing or caring.

This is a unique advantage to WCs. And it's such a compelling advantage that it makes almost all the downsides tolerable. At least if you are in a position where your component might be used in many places. Design Systems are a good example. The consumer can be any app, built on any technology.

And there are plenty of downsides. But they are mostly felt by the author, not the consumer. Similar to how TS libs can be very complex to author because of type level gymnastics, but when done right are easy to consume.

(I do not push them heavily, but I can appreciate strengths and weaknesses of all tools)