Semantic is pretty cool. The advantage that Treesitter has (shared with LSP servers) is that it's client-agnostic. So while no one besides fellow Emacs users are going to hack on the Semantic parser for a given language, Treesitter can potentially draw help from users of any editor, or even people doing other things like building analysis tools.

Semantic is used less and less I would say, there is not enough people involved in keeping it and it's grammars updated, it also suffers lack of asychronicity. Tree-sitter and LSP has the advantage of re-using code for multiple editors