Hack does have a lot of great features (some of which have already been ported to PHP, others are kinda impossible for technical reasons), but at this point, it

Hack does have a lot of great features (some of which have already been ported to PHP, others are kinda impossible for technical reasons), but at this point, it's diverged so hard from PHP that they're separate languages. If you choose Hack, you lose the entire PHP ecosystem (composer) that could back you up and save you time. The only ones really using it are Facebook/Meta, so it would be a mistake to choose it for a new project, IMO. It doesn't really have a future.