The per-unit energy is the biggest issue. It's true that fire is a problem for batteries, but that is manageable with very small sensors and casings. There ar

The per-unit energy is the biggest issue. It's true that fire is a problem for batteries, but that is manageable with very small sensors and casings. There are many more systems required to deal with the many failure modes of internal combustion engines: - Heating systems to address carburetor icing. Typically manually activated by pilot since they have a performance cost. - Fuel pumps since gravity cannot always be depended upon especially in acrobatic airplanes. - Fuel sump systems which must be checked before every flight to address water in the fuel tanks & lines. - Lubrication systems which require pre-flight fluid level checks. - Magneto system to ensure that sparks can be generated independently from the electrical system. - Redundant magneto system since carbon tends to build up and prevent proper sparking. - Possibility of an engine running rough/cylinder firing out of sync due to carbon or other issues. - Fuel/air mixture control so that the optimal mix can be selected based on altitude. Requires pilot intervention. - Extreme thermal stresses on the engine, requiring engine run-up checks before takeoff. - Radiators, and airflow rediretion to address cooling issues resulting in loss of performance and weight. - Exhaust system, and noise vs. weight tradeoff