And this is false for Haskell: it's not enough to know language syntax
(it's relatively simple): each library can involve own level of
abstraction, own eDSLs, etc. And if somebody built his library on
arrows, pipes, free monads, etc - it's not enough to know language's
syntax only. Imagine a big house built with simple and same bricks. And
some Baroque theater where anything is complex and unique.
So, languages like Haskell are more complex and need more time to learn
and create valuable applications.
OO has its own version of this… more insidiously. You're prone to see a class hierarchy and think you understand it up front because it's all familiar things, but every application in effect has its own distinct notion of what a given class means, exposed as either custom methods or custom implementations thereof implementing the app's specific logic. Haskell's FP style makes you expose this directly. (But it's the same amount of complexity underneath, so ultimately not really different; it just taxes the programmer in different ways.)