Framework server rendering is framework specific... Seems like a framework problem.
At least custom elements can just be including in framework-specific templates and be included in the rendered output, even if not deeply server-rendered.
It's actually very difficult to do this correctly and at scale with a framework like React, because in React you need an ID and JS call to initialize each embedded component, when custom elements initialize themselves.
Anyway, you have an axe to grind, so have fun with that. I'll see you next time I make a positive comment about web components I guess.