The first part of the paper shows the advantages and disadvantages of using GaN and SiC substrates with an off-cut up to 2 degrees. The following experimental observations were made: i) higher efficiency in p-doping with magnesium, ii) higher critical conditions for AlGaN cracking, iii) trigonal deformation of AlGaN and InGaN unit cells. A successful growth of AlGaN/GaN HEMT epi structure on 2 degree-off SiC substrates will be shown, what paves the way to a monolithic integration of GaN-based and SiC based devices on a common SiC substrate. In the second part of the paper, we show the experimental results on AlGaN/GaN degradation upon prolonged annealing at 600oC. In the case of epi structures on Si and sapphire, the parasitic parallel conductivity appeared, whereas in the case of SiC substrates, annealing introduced no distinct changes.