No, HM algorithm has terrible worst case complexity, but the worst cases don't correspond to programs humans write and generally it works perfectly in practice.
IIRC, the algorithm used by the ocaml implementation is guaranteed to be close to linear time as long as you dont have this sort of nested polymorphic lets.
In my experience the failure rate is far below 5% for vanilla HM algorithm. I've never seem a pathological input in the wild that wasn't contrived specifically to test the typechecker.
