I'm going to guess you'd have more software complexity to account for more dynamic conditions, as OP's link alludes to, along with more redundancy in safety. I'm pretty sure most 4 motor drones can function fine with just three blades/motors and possibly two even.

Source: I'm an expert flyer with dozens of seconds flying. I once bought my kid a tiny 2" drone and eventually took over as pilot. Now I didn't take it from him, he just stopped playing with it (What's wrong with kids these days? I would've loved it as a kid) but those little things are so fun to fly.