Fixed wing aircraft are more efficient than rotary wing (quadcopters, helicopters, etc) in forward flight but are less agile and therefore not capable of landing without suitable infrastructure such as runways. This lack of agility in fixed wing aircraft limits their applications.
Combing both propulsion methods into a hybrid propulsion system results in one or both systems to be inefficient to a degree where the aircraft becomes unfeasible.
This is known through some basic physics relating to airfoils, for instance;
A large diameter, low pitch, slow rotating, 2-blade, hummingbird shaped propeller is optimal for hovered flight.
For forward flight, greater velocity correlates to more blades per propeller, more aggressively pitched blades, faster rotating blades.
Optimising efficiency can allow for a feasible novel VTOL aircraft by combining and optimising both propulsions systems.
The main concern is a resulting increase in complexity with regards to the issue of having different propulsion systems working together.