Basically the problem with n810 is the packet injection, which is not possible. And if you have a laptop do to the injection, well, you could do everything on the laptop. I guess n900 won't be able to do packet injection either, but that would deserve a check.

Another point is that the n810 doesn't have too much power to calcul stuff, and cracking wireless encryption might be power-hungry.

Oh and, depending on why you want this (is this for surfing freely or for doing professional penetration testing?), nxx0 might not be the perfect device.