Standup in mma is at such a low level - and has been since the beginning. however i do think it has made progress in the past 10 years - anderson silva is an example of that progress. people are in awe at what he did to forrest, but if you knew boxing, you'd know that any good boxer with a certain style could toy with forrest like that. i'm glad silva openned some people's eyes to that.
there is more money in boxing for the fighters.
there is a deeper talent pool in boxing.
those are facts.
those facts lead to the conclusion that it is more difficult to succeed in boxing than it is in mma.
sure, in mma you do all disciplines of fighting and whatnot. but the talent pool is shallow in comparison, the level of competition is not as high - and those are the key elements that determine the difficulty of competing in a sport. mma is simply not developed enough to be more difficult to compete in than boxing right now.
to say mma is tougher JUST because it is more varied is flawed. that's like saying being a jack of all trades is more difficult than being a master of one - that's not necessarily the case.
the fact that high school teachers, WWE wrestlers, engineers can just leave their occupation and compete in mma at the highest level within a just a few years (after their physical prime btw) is TELLING.