I believe that to be mostly true for Lisp or ML-family (OCaml, ...) languages, but not for Haskell. Reasons would be: - lazy evaluation can be tricky if you work with files, you have to profile and force strict evaluation at key points to avoid memory explosion. - refactoring something to work with the IO monad is hard - typing some higher-order functions is hard

It may be subjective, but even for the syntax I felt a much stronger cognitive load working with Haskell.