There are no polymorphic literals in ML, just polymorphic math operators, which is enough of a blight on the standard that OCaml discarded it and forces you to use different operators for real and integer arithmetic. And there's only one kind of string in both MLs.

Haskell's Num hierarchy is troublesome. They traded usability for + with complexity for /. It's extremely unlikely that you could write a program in Haskell that does much arithmetic and have it build correctly on the first try without any manifest typing. This is one reason students of Haskell find things so confusing: type declarations are necessary at the top level simply because the extensions and complexity of modern Haskell break HM if you try using it everywhere. Also, the class system in there is not especially mathematically correct, which leads to the numerous replacement Preludes that try to do a better job but haven't caught on.

Strings are edge cases because they are not polymorphic unless you enable OverloadedStrings. Once you do, you will either replace the built-in string with something else (ByteString or Text) or find yourself in the same kind of trouble you'd be in with Num.

Let me be clear: I'm not saying that these problems are showstoppers. They're really minor annoyances once you're experienced, though they contribute to confusion for beginners. The point I'm trying to make is that you can't just drop HM into any old language and expect it to work. A greater point would perhaps be that all languages have warts simply because they're large, complex beasts (Haskell and C++ especially) and it's unproductive to point to a missing feature in one and demand some sort of perfected version of the other's.