The main advantage is when doing something like this is Haskell the code you pass around can't have arbitrary side effects, so you don't actually need to worry so much about where it is called.
Yeah, I agree that having no side effects helps quite a bit. But you still need to understand control flow to reason about time and space complexity, which are a sort of "unavoidable effects".
If "it is almost impossible to understand control flow in Haskell" as you posit, then how are so many people making time and space efficient Haskell libraries?