This study demonstrates the advantages of using a hybrid compliant substrate containing a porous silicon (porSi) layer, obtained through electrochemical etching of the original silicon substrate, and a silicon carbide (SiC) layer, formed on its surface through the Kukushkin method, to grow high-quality, ultra-thin nanostructured Al_хGa_1-xN/GaN heterostructures with nanocolumnar morphology by molecular beam epitaxy with plasma-activated nitrogen. Comparison of our experimental results with information from prior literature illustrates that the use of such a hybrid SiC/porSi substrate has a number of undeniable advantages for the growth of ultra-thin Al_хGa_1-xN/GaN nanoheterostructures without the use of thick A_IIIN buffer layers. Direct growth on a hybrid compliant substrate and SiC/porSi leads to a substantial relaxation of elastic stresses between the epitaxial film, porous silicon, and silicon carbide; this consequently affects the structural quality and optical characteristics of A_IIIN nitride-based transistor ultra-thin structures. The experimental and computational data obtained in our research are important for understanding the physics and technology of Al_хGa_1-xN/GaN nanoheterostructures, and they will contribute to their potential applications in optoelectronics.