Watch the Mixed Mode Harmony online guitar lesson by Matthieu Brandt from Songwriting on Guitar
The amount of toughness of composition using mixed mode is dependent on several factors. Introducing the bIII, bVI and bVII chord in a composition in a major key will add a certain amount of edgyness. That feel is strengthened by using these added chords in each others vicinity. By chosing melody notes on top of these added chords that are unique to them (and not present in the original major scale) you will stress the rawness even more.
Our ear has gotten used to these raw sounds in a major song and can handle them. Often these chords are used to create contrasting sections, but many songwriters have made them an integral part of their songs.