The jsx is there primarily for better typescript support and react-like api.
Internally it compiles down to template literals, so the same advantages apply.
The typescript support for lit is lagging to say the least. It is surprising that there is still no good official support for type-checking of templates. There are a few community projects though but they are not as reliable and the DOM/Web Component API doesn't make it easy to make fully type-safe APIs.
Solid being able to lean in the JSX support in the TS compiler and not tying itself to custom elements api is able to offer a much better DX here.
No, solid.js is building reactive graph at runtime and in theory should be able to also detect static inputs at runtime (not sure how much effort he put into reactive graph optimization techniques). Personally, nowadays I prefer S.js/solid.js approach, but it has different tradeoffs, like it is essential to understand the difference between solid.js and React/Svelte/lit/etc :)
It is an old idea with reactive graph created at runtime and direct bindings (knockout.js, etc), but as always, implementation is way more important than some abstract idea, and ~6 years ago Adam showed that it is actually possible to implement this idea quite efficiently (S.js+Surplus). Then Ryan started working on Solid.js and it became one of the most popular implementation of this idea.
There are a lot of things that I don't like in Solid.js implementation, like it seems that he still don't care about performance in general and only focuses on getting high score in js-framework-benchmark (optimizing library for two cases: DOM template cloning and one/many-to-one reactive bindings). But I believe that it is not something that is inherently wrong with an idea and there are a lot of room for improvements in implementations.
I guess the main tradeoff with such idea is that it has a slightly higher learning curve than something like React with its top-down recompute/rerender approach (as long as we don't care about performance). But when we start to add reactive systems to react/svelte/etc to improve performance, at that point it becomes more complex than just using UI library specifically designed for reactive system.
Right now I am trying some experiments with new algorithms and datastructures for reactive system, that I specifically designed for UI problem space, to actually beat vdom implementations in microbenchmarks that were heavily biased towards vdom-like libraries (reimplementing top-down dataflow+diffing in reactive system with derived computations, it is super useful when building something like https://lexical.dev/ )
EDIT: Also, in such libraries it becomes quite hard to implement features like reparenting or DOM subtree recycling. But it seems that nobody cares about reparenting in web UI libraries (Flutter supports reparenting). DOM subtree recycling is quite useful in use cases with occlusion culling (virtual lists), but it should be optional with different strategies to reclaim memory (not how it is done in Imba library).
