I would blame it on the control system and the engineering of the vehicle. I don't think the number of rotors is a big issue... each cluster of 4 props could be operated in unison, making the control system see it as a quad-copter.
Smaller electric multi-rotor aircraft run into the same stability limitations to a lesser degree, but it can be compensated for with an appropriate set of PID variables. The issue is the mass of the rotating assembly vs. the torque available to accelerate it. Electric motors have gobs of torque, so it's not as big an issue.
Internal combustion engines have 3 methods of boosting torque. Increase piston stroke (the length of the crankshaft throw, increasing leverage), increase piston bore (the diameter of the piston, which increases surface area on which the expanding air/fuel mixture presses) - both of which increase the displacement and overall size & weight of the engine - and finally, increase the pressure of the expanding air/fuel mixture by increasing the compression ratio of the engine and/or moving to forced induction. All of those options have tradeoffs in terms of weight, reliability, mechanical complexity and cost.
The best solution would probably be to use an engine tuned to operate at a specific RPM and move to variable-pitch props. Rather than change the speed of the props and face the associated acceleration lag, simply increase or decrease the pitch of the prop to achieve the desired level of thrust.