1. Physics

The rules of the natural world are unbending, and when people try to bend nature to their desires, it's the people and not the rules that get bent. Physics dictate that to lift something, you need to exert a certain amount of force. The heavier the object being lifted, the more force you need. When it comes to what one might charitably call an "emerging" segment (the term implies that it's actually going to arrive at some point) of small multicopter and tiltrotor craft, weight is the enemy in more ways than one. For the plane to be both mildly crashworthy and to have a reasonable level of endurance, let's say 45 minutes of flying time, the craft would need to have fairly powerful motors with plenty of juice. Small ultralights with HUGE wings can fly reasonably well with 40 horses of power. For a mostly vehicle thing with no wings or tiny ones, that figure would need to be much greater, likely at least 100 horses. If it's not powered by an internal combustion engine, then that craft would need a lot of batteries, which are far, far heavier per unit of energy than a gas piston engine is. For hybrids, add up all the engines and motors and batteries and the weight penalty is staggering, which is a good word to describe how one of these things might fly, staggering into the sky.