Hack might be better from a language design point of view, but most people outside of Facebook seem to have lost interest in it once PHP 7 caught up to it performance-wise.
You still can mix it with plain PHP, much like you can mix TypeScript with plain JavaScript.
I wish high-profile PHP projects, like Nextcloud, migrated to Hack eventually; it can be done piecemeal.
Have you used contexts/coeffects?
My understanding is that it's still yet to be fully rolled out to WWW, and nobody outside FB has yet played around with it. IMO it still has some DX issues (https://github.com/facebook/hhvm/issues/8828) and the examples given in documentation don't actually work in real life (https://github.com/hhvm/user-documentation/issues/1016).
It's definitely a fresh feature, iirc rolled out last winter.