[en] We investigated by using high resolution x-ray diffraction and glancing incidence x-ray reflectivity In_xGa_1-xN/GaN and Al_xGa_1-xN/GaN multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy on (0001) 6H-SiC substrates covered by a GaN buffer layer. In particular, the macrostrain field is investigated by using a theoretical approach based on the exact calculation of the incidence parameter and an arbitrary strain field and it is directly assessed by measuring the angular separation between the heterostructure and substrate Bragg peaks for several reflections and diffraction geometries. The experimental results reveal non-zero off-diagonal strain components for all the investigated samples, indicating a triclinic deformation of the buffer and the superlattices unit cells. Moreover, we found a coherent interface between the buffer and the Al_xGa_1-xN/GaN MQW, and a partial relaxation between the GaN buffer and the In_xGa_1-xN/GaN MQWs. The chemical composition of the ternary alloys has been determined from the strain components by assuming Vegard's law and applying the equilibrium conditions of the elasticity theory. We found a pronounced In-segregation if the MQW was grown under metal-stable flux conditions; in contrast no appreciable segregation effect was observed under nitrogen-stable flux condition. A cumulative interface roughness was found for the samples grown on nitrogen-stable flux condition, exhibiting a strong increase of the rms interface roughness towards the MQW surface