Gas engines generally lack the immediate throttle control/thrust response necessary for use in a multicopter. Why didn't we see gas RC quadcopters before electric ones? My sniff smells ok.
Would you want a centralized gas engine powering your 4+ rotors causing them all to fail at once or would you like the complexity of 4+ separate ICE engines trying to work in concert with precise torque output?
Solar panels are incredibly thin and light. There is no reason not to include them.