In the end, more moving parts generally equals less reliability.
And this, has many more moving parts than a traditional wheel.
I'm not saying that it is going to be 6x more prone to breaking down than a traditional wheel, or that if one of the side-rollers breaks that the whole thing locks up and becomes dangerous. I am saing that there is a much higher chance of a bearing failing at one point or another, requiring a replacement for a return to the original efficiency.
And cnnrmccloskey: They use a regular hub to drive the cog of the wheel. Plus, each of those rollers has bearings of its own.