-vibration, particularly for IMUs
-dirt
-safety during autopilot failure
-noise during long-term repetitive overflight of any given target
-problems with air mixture at high alt
-problems with unreliable throttle
-usually need parachute due to total weight
-longer time from deployment to takeoff
-almost no positive results on known public UAV forums
Also if you're going to use a gas engine you're going to have to use shielding because of the interference coming from the spark plug. I've almost lost a plane to spark plug interference, it rebooted the arduino in charge and left it with elevators maxed down. Placing the board further from the engine fixed that.
Gas engines introduce more vibration, which means that your IMU won't perform as well. But it shouldn't be a show-stopper. And of course ArduPilot, which uses thermopile rather than an IMU, performs as well we gas as electric, as long as the exhaust of the gas engine isn't going over the thermopiles.
Replies
-dirt
-safety during autopilot failure
-noise during long-term repetitive overflight of any given target
-problems with air mixture at high alt
-problems with unreliable throttle
-usually need parachute due to total weight
-longer time from deployment to takeoff
-almost no positive results on known public UAV forums