Single engine, single-rotor aircraft like helicopters are inherently more dangerous than a multi-rotor craft like an octacopter simply because of the design --- a single engine failure causes a crash, whereas an octacopter would require two or more motor failures and/or rotor failures --- it's the same reason commerical airliners can operate on a single engine in case one or two engines fail --- can't do that with a single-prop plane
I am assuming you are not a real pilot because of your statement …"A single engine failure causes a crash". This is completely untrue. A pilot that experiences a single engine aircraft, engine out situation, be it rotor craft or fixed wing, that is unprepared, has not been paying attention to the ground he was passing over and not flying with any thought to what would happen if the engine stopped, will be the one that ends up in a crash.
If you are flying the way you should have been taught by a competent instructor, you will always have that thought in the back of your mind that the engine might one day fail, so what will I do. When you fly, you pay attention to the ground, watch up ahead as well as to the sides, picking out spots where you could put it down in, if the fan stopped blowing. You remember the spots and don't forget the ones you just passed by that are now to your rear. You fly at a safe altitude that gives you time to trouble shoot an engine out situation, in case you can get it restarted and you go into dead stick landing mode when all goes silent.
The aircraft will still fly, you just adjust your settings for best glide, have in mind where you will be putting it down, scan the sky and go about setting up to put in that spot you have picked out. It does not just stop in the sky, then drop straight down, like it does in cartoons. The moment you throttle back any aircraft, you are then without power to continue level flight, so effectively gliding, all be it with some residual power there, but an airplane does glide without power, some better than others. Look at Sully, he glided his aircraft right into the Hudson river. The space shuttle glided all the way back from space to a safe touch down each time, with no engine.
A rotorcraft can also fly when the engine stops because the main rotor is freewheeling or windmilling as you drop. The pilot would go into auto rotation mode, pick a spot to put it into, then set it up and just wait. At the correct altitude, he would go through the motions he was trained to do and bring it in to a bit of a bumpy landing but none the less a landing. The tail rotor only works to offset the torque of the engine when it is running. When it stops, there is no more torque, so no need to be as concerned. If you lose a tail rotor in flight, you would immediately shut down your throttle and go into auto rotation, but that needs some very fast reactions. All pilots should have been trained in engine out scenarios, so that you hopefully will not panic and just crash. Now not all outcomes are good ones, as we all know and can possibly be due to the way the pilot was flying and possibly because the pilot became complacent, thinking their engine was never going to stop in flight. But please don't think that just because the engine stops you will be crashing because it is all down to the pilot, not the aircraft in that situation.