I am not sure about this in particular. But in general for redundancy to work you need the remaining equipment to be able to support the craft fully. There nee

I am not sure about this in particular. But in general for redundancy to work you need the remaining equipment to be able to support the craft fully. There needs to be enough spare power. And the remaining thrust must be central enough to avoid loss of control. This doesn't work well with even numbers like 2x or 4x. Also, adding additional rotors/engines could increase the chance of a failure in a particular flight as you have more things that can go wrong. A 2 rotor helicopter is definately not safer than a single bladed one.