I've often heard that although combi microwaves have an oven function, you cannot bake cakes in it. Mainly because it doesn't build up enough heat. If you increase the temperature - the cake crust gets burnt before the inside finishes baking. If you use a low temperature - the microwave times out and automatically shuts down to cool off.
You can make regular cake recipes in a combi microwave. Firstly, use the highest rack the oven comes with, so the cake pan is closest to the source of heat. Secondly, amend the cooking times to suit the combi microwave. If the recipe calls for 350 degrees F for 30-40 mins, then bake the cake at 200 degrees C for 40 mins and 180 degrees C for 30 mins. Cake comes out perfectly all the time. Happy baking!