The level of striking in MMA doesn't show what is the hardest to learn. It shows how much more important wrestling and bjj are than striking.
Yeah the level of striking in MMA is actually pretty straightforward. It is probably not even 1/10 of the entire body of knowledge of striking (if such entire body of knowledge exists).
Just look at Muay Thai, Boxing and sanda (sanshou) - how many of those techniques and tricks were not utilized by MMA fighters (not to mention that in some cases those MMA fighter's striking techniques are not even very good if you are to look at it from a striking only standpoint).