They criticise plastic controllers, now they criticise real guitars. Where's the happy in-between?
Those opposed to the music game genre generally believe the lack of realism is holding it back. It doesn't take any sort of musical skill aside from a good sense of beat to push 5 coloured buttons. Yet the instant it requires a real instrument, and real playing ability, it's not about the controller at all - you're still playing a game which immediately destroys any sort of credibility the activity of playing an instrument may have associated with it.
How is this any different to playing along with a CD, the radio, or youtube? It's not. It's arguably better as it will provide feedback on your progress while giving you a genuinely entertaining way to learn (by experiencing true rock and roll culture) rather than the stale "these are chord charts; now play these scales" you get from your local guitar school/tutor. I also hope Ubisoft will include an advanced tutorial for improving your playing technique instead of simply repeating a section of the song until you can perform the require button mashing from muscle memory.