What are the drawbacks of this compared to coaxial configuration with a turboshaft engine?
Stan Hiller's earliest helo designs were coaxial and used relatively small 4-cylinder aircraft engines to drive the coax transmission, so it is indeed possible to power a small coaxial helicopter with a relatively small piston engine.
Hiller also found that autorotating a coax main rotor blade setup was difficult and so his later designs used a more conventional single main rotor blade and a tail rotor.
Sure. Earliest Kamov production helos - Ka-8 and Ka-10 - were powered by piston engines.