It's not that the general idea of "gradual typing" is bad, but that strong types become a leaky abstraction in practice. If you have to constantly fight against the weakly typed nature of the underlying language, that defeats the point.

Here's a writeup on it. https://www.onux.com/jspp/tutorials/type-system#Appendix-B

The problem is not as simple as taking the ECMAScript grammar and augmenting it with type annotations. There's a reason that Microsoft (TypeScript, Safe TypeScript), Google (AtScript, SoundScript), and Facebook (Flow) have all collectively attempted this problem and came up short.

It continues for a while...