There are essentially two types of VTOL: hybrid and vectored. All other VTOLs are a variation of the two. One is far easier to build, and one flies much better (and more efficiently). Unfortunately though, these are not the qualities of the same type of VTOL.
This is certainly easy to design and build. It's just a matter of taking a very good airplane and attaching a multirotor frame to it (centred around the CG of the airplane). That's a bit of an oversimplification, but accurate. Unfortunately, with this type of design the multicopter propellers themselves can begin spinning during forward flight due to the wind. And even with no spinning they are essentially control surfaces and can cause strange turbulence and vortices around the wings, which alters the way it flies. This, plus having surfaces designed around moving air that are non-functional during forward flight, will reduce your efficiency. And (in general) making a fixed wing drone is about trying...