Take a look at simulationcraft. It is the base for raidbots and others.
I used it initially to check the DPS and tanking capabilities of a class with talent and gear combinations. It showed me many times how bad a player I was.
Then I got into writing my own scripts, and that leads to really learning the class well. Else you cannot pull the DPS.
Based on that experience, I can say that I personally do not have the visual and brain processing acuity, to compute all what is needed to be able to choose the right abilities in real time. I will make tons of mistakes and I will end up spamming, all the while losing DPS.
Yesterday I was messing with Fury on the PTR, trying out the new talents. I had a dozen buffs flashing (and shifting) on screen, my actionbar was lit up like a Christmas tree, and I had to make decisions: Onslaught, Raging Blow, Rampage, Execute, Odyn’s Fury or maybe Bloodthirst. 5 talents just on Bloodthirst… Could not even find the Enrage buff hiding amongst the dozen other buffs. Let alone what those buffs all are and how they affect the rotation.
Therefore, in my opinion, players who do the Mythics are very, very good. The rest think they are, but are not.