When you turn any engine "upside down" you complicate oil scavenging, make air/oil separation more challenging, and increase the risk of hydraulic lock in the cylinders as the engine cools down. More a problem in air cooled engines with their looser tolerances, but liquid cooled engines are not immune. That's why "inverted" engines are in the minority everywhere except in the lands of DeHaviland, Daimler, and Junkers.Why not? What is "inverted". In aero engines a huge number were rotary or radial. Some are horizontally opposed today. The convention of V engines being with cylinders pointing up comes from the car industry and modern times in the 1930s there were no such conventions.
Cheers,
Wes