Sports require different skills, like endurance, speed, reflexes, etc. Nobody can say that any of those are "harder" than the other because they're all extremely difficult to work at perfecting, and how hard that is is something defined person-to-person. People might suck with endurance but can run short lengths extremely quickly, it just depends on what's hardest for them. As a whole I don't think you can really define anything as the "hardest" for that reason.
Though personally I do agree that certain types of MMA are probably the most complicated and difficult to master, because rather than a sport, most of them interconnect with your daily living, and require a new type of lifestyle and discipline to truly get the most out of it.