Except the problem with compatibility is almost always the reactivity element, right? Getting, say, Vue's reactivity system to compose properly with Svelte's, or React's with Angular's. And that's not going to work well when Vue is using signals to decide when to rerender a component, React is using its props and state, and Svelte isn't even rerendering components in the first place.

This is especially difficult when you start running into complicated issues like render props in JSX-based frameworks, context for passing state deeply into a component, or slots/children that mean reactivity needs to be threaded through different frameworks.