Will the 6D require somewhat more skill and planning to get the shots you want compared to a 5D MKIII? Yes.
You're talking of focus & recompose skill with thin dof, I guess?
One thing I'm experiencing with my 100L and the 6D and I have "learned" to circumvent by taking more frames is that the 6D sometimes produces actual af misses, i.e. the focus is completely elsewhere even though the camera did an af confirm (red dot) - I practically never experience this with my 6d af, though the 60d more often has "micro misses" while the 6d is either spot-on or lost in the woods.