Is complex villain necessity to write good heroic story

And this itself proves his point, that even a "Force of Nature" can be a "villain" for a "hero" to overcome if written properly. It doesn't even need to have much of a personality. The other example I can think of is Moby Dick. The White Whale doesn't even say anything, he just exists but still carries the entire plot of the book.

The Old Man and the Sea is another. You could even say Robbinson Crusoe has Nature as the villain. 

90s Disney animation like Aladdin and Lion King use Joseph Campbell's Hero with Thousand Faces as a guide. Heroic stories don't necessarily need complex villains to be great, but they need solid character arc for the protagonist. 

I really depends on how you want to write the plot. If you want an action-packed battle story, where fights make up the majority of the plot, then a complex villain isn't necessary; however, if you want to have a story with a plot that delves into the subtle aspects of the human (or otherwise) psyche, then it would be better to make, not only the villain, but many of the characters have multifaceted personalities.

As an example compare Fullmetal Alchemist manga VS Kuro no Shoukanshi manga. In FMA, all of the battles serve a purpose in the secret plan of "Father" vs in KnS, so far, it's a narcissistic psycho and a scheming glasses bastard. 

I don't think this is necessarily true either.  A complex character doesn't necessitate that he be sympathetic.  It's entirely possible for a good writer to construct a complex multi-faceted character who is still utterly despicable. 

Except villains can be protagonists as well. 

Hence the open inverted commas. They both serve a similar function. In fact, you can say villains are a subset of antagonists. 

Villain and Antogonist are not equal to each other. Do not mix them. 

Depends on what the story is about.
Villain can exist only to create situations for a main character to resolve (for a comedy for example)
Villain also can be a complex character and the reason he is a villain can be that his motivation and reasoning is different from the hero's and so they are in direct confrontation because of that