Is hybrid propulsion widely used by UAVs?
It's becoming common, particularly for endurance.  Two such commercially produced aircraft, Skyfront's Perimeter 8 and Avartek's Boxer, have several hours of hover endurance and a 5 kg payload capacity.
Academia is also taking note: here's a 2018 master's thesis that analyzes the advantages and challenges of hybrid power.
The core motivation for hybrid power is that the internal combustion engine can be optimized for nominal power, while the battery need only handle the difference between nominal and peak power.  That weighs less and costs less than a big IC engine or a big battery, while still enjoying the greater energy density of IC fuel as compared to batteries.
Compared to purely electric propulsion, two challenges are the engine vibration that corrupts sensor readings, and the louder noise that constrains where the aircraft may be flown without annoying others.