Many non-trivial libs are typed improperly, even when they can be done right. There's massive abuse of "any" type the moment anything gets semi-challenging. Also, a non-trivial amount of type definitions don't work in strict mode, which make things worse.

Anything involving complicated higher order functions (like lodash/ramda's curry), or having a single method that can be used in various ways that don't fit well with TS' overloading mechanisms are often done wrong or poorly. Flow is a bit better at this, but the techniques it provides are not always well known, so the type definitions aren't that much better.

Well, there are over 10,000 occurrences of 'any' (which effectively gives up on type checking) in the DefinitelyTyped repository:

https://github.com/DefinitelyTyped/DefinitelyTyped/search?q=...

It's true it's easier now that typescript has richer types (nullable, mapped, etc)