It's more a function of language complexity than anything else. LSP for something like C, Java or TypeScript is not that hard, because the language is fairly straightforward to parse, and not ambiguous. But something like C++ or Rust, where you have to deal with templates/generics/traits/... and type inference starts getting gnarly. And something like JS or Python is hard because there are no static types at all, and you have to do inference.

But LSP itself doesn't really make it easier or harder. If you look at other code completion implementations (in IDEs that don't use LSP), they're usually at a comparable level, if they exist.