Apparently, it's a little bit easier than Dark Souls using a Rock Band controller.
Mainly because there's less platforming involved, and less chance of falling to your death. It took Twitch streamer Bearzly nine hours to beat the game using the Rock Band controller and it was only his fourth playthrough. You can watch the whole thing here at his Twitch page.
For reference, here are the controls he used...
Human beings are capable of achieving incredible things. We've landed on the moon, we've built towers that stretch towards the sun and microprocessors smaller than the eye can see. But it all pales in comparison to the ability to play difficult video games with a Rock Band controller. Those people are the real heroes.