The three major/three minor" chord system that Jonny Sunshine claims is "straightforward" really isn't--A typical melodic phrase is four measures-so with only on chord change per measure, there are 1296 possible chord progressions.

Only thing is, there are seven chords, one triad based on each scale step, not six--so that means that there are 2401 possible chord progressions, and that's if you only allow one change per measure--and we haven't even gotten into talking about modal scales yet.