> Functional programming languages shine when they deal with complex, recursive data structures, not for simple tasks which are primarily IO and need to handle many cases.

I think most of the advantages of FP languages (as they exist today) become disadvantages as your data structures get more and more complex. Efficient mutability becomes necessary. Pattern matching breaks down as (G)ADTs can no longer encode useful invariants. Referential transparency becomes useless once you realize you want explicit sharing. Laziness and space leaks become a huge PITA. The RTS and the mandatory GC start getting in your way as it gets harder and harder to eek out performance from your code. etc etc..