Exactly. Functional programming may or may not be "worth it" to use for one particular problem or another.
However, if we ask ourselves whether functional programming is worth learning (as I suspect the poster is really asking), then the answer is clearly yes, if for no other reason than that's the only way you can make an informed decision as to whether it's worth applying FP to that particular problem of yours.
I agree. It's a myth the functional languages are simply more difficult. Once you learn to think functionally, it's just as natural as writing imperative code and doesn't take any more time. In fact, once you get really comfortable with the abstractions, you can move much more quickly, through complex recursion and all.