[JshWright]

A quadrotor doesn't have any sort of 'redundancy'. If you lose a rotor, you've lost control, and will be hitting the ground in pretty short order. I see multiple rotors as a disadvantage... more things to go wrong.

[JoshTriplett]

That depends heavily on your control system and how well you've tested it. In theory, a control system for a quadcopter that has independently controllable rotors could keep itself stable if one goes out. In practice, any control path you haven't tested probably won't work.

[JshWright]

The biggest issue I see is dealing with the unbalanced torque... You could probably manage a 'controlled crash', but that's about it.

[murbard2]

It depends on your design. You can build it with separate engines and manage to land safely on two diagonally opposite engines.

[kenrikm]

I'm not sure that's feasible. I've been flying multi-rotors for quit some time and if you lose a motor on a quad it's nearly instantaneously on a collision course with the ground. Y-6 or Hexacopter would be more feasible from a redundancy standpoint.

[derwiki]

Quadroter no, but multirotor yes. DJI's S900 and S1000 are designed such that two props can fail and it stays in the air.