Functional programming? I haven't honestly had a chance to use it long enough to find it hard: every time I've worked in a group that was even willing to give it a chance, everybody else gave up before I had a chance to make up my mind whether it was harder/easier than OOP. I personally kind of like the functional aspects of Java, Javascript and Python (although I still don't have the first clue what a "Monad" is).

I've been working primarily in Java for the past 20 years, across four different companies, and as far as I can tell, most everybody gives up even on OOP, too: every system I've ever worked on that was written by somebody else was 90% static functions & static data, with a few "dumb object" data types just because that's what you get from JAXB.