Yes, of course. But toy helicopters have been around for a lot longer than quad-copters have been popular. And I've not seen a lot of people using single-rotor

Yes, of course. But toy helicopters have been around for a lot longer than quad-copters have been popular. And I've not seen a lot of people using single-rotor RC helicopters for filming and delivery (I'm sure there are some ...). My point is that multicopters are a lot simpler mechanically. No articulation, no transmission, just propellers on an electrical motor, controlled by pure electronics with no mechanical components in the loop except the drive coils. Arguably multicopters are easier to control automatically (with modern electronics), though that wouldn't be THAT big a factor today, I guess. Also electrical engines can deal a lot better with variable load and RPM. Multicopters with combustion engines (driving the rotors directly, not through a generator) seem to be quite uncommon. Still, if multicopters are supposed to be routinely used in the presence of people, they need similar standards of maintenance and failure control as would bigger aircraft, be they manned or unmanned...