I use it for everything and love it! I never mix it with anything, I just use it straight. I have covered cakes with MC panels, molded figures, used it for cutouts… I bet your recipe needs tweaking. I have tried Mike McCarey’s and Baking Arts recipes and had great success. I don’t have them in front of me though. If you can’t find them online and want them, let me know.
I used modeling chocolate exclusively to make my Bonsai Cats Threadcakes entry this year, if you want to see a good sampling of everything it can do.