Especially when there is sauce in the food. It seems the sauce heats less evenly in the microwave. I suspect the kind of flour used in the sauce is the problem but am not sure.
I believe it has more to do with HOW a microwave cooks or reheats food vs. the way a conventional stove top does it.
When you put something into a microwave to reheat it, it does not apply heat the way a stove does. Microwaves use their namesakes -microwave radiation- to jostle the molecules contained within the food, causing friction, which in turn causes heat, and heats up the food, from the inside out. This means that food with a higher density, such as meat or veggies, will heat up more quickly than liquids, which are less dense. Also, microwaves don't just heat up the food, but also the container, be it plastic, foam, or the stereotypical white paper boxes. When this happens, the particles within the container (which are being heated) will give off various chemicals, which can alter the taste of food.
When you reheat food on a stove top, you are using a container that is much more resistant to heat (a pan) and you are applying heat evenly, from the outside of the food, regardless of it's density. This means that the pan (90% of the time) will not alter the way your food tastes, and because everything is being heated at the same time and rate, your food tastes more like it did yesterday, or whenever it was that you brought it home.
There are probably several factors which lead to the perception that chinese food heats less evenly in a microwave than on the stove top.
There are lots of types of Chinese food, but due to the mention of sauce on the original question, I am going to assume its a dish with meat and vegetables in a sauce, like (as often available in US Chinese restaurants), Kung Pao chicken or beef with broccoli. Since the liquid is thickened, and only part of the dish, little convection can occur--transfer of heat within the food itself will be by conduction unless the dish is stirred.
Contributing factors probably include:
All of this leads to the main probable cause: less stirring. You are probably more likely to be actively stirring the food on the stovetop, where the heat only happens at the bottom, and may burn on the pan, thus distributing the heat throughout the dish.
With the microwave, stirring still is important due to all of the factors listed above, but you have to stop the cooking and open the oven to do it so it possibly doesnt' happen as much.
Truthfully, I find the microwave the superior method for reheating Chinese food. Try these suggestions to make it successful:
It depends on the dish. Reheating Mongolian beef or Kung Pao chicken in a microwave does not produce a better result than reheating on the stove, but reheating General Tso's chicken or sesame chicken does. The latter two turn soggy or slimy or both.
(My opinion, as I'm not really sure):
Have you ever put a piece of bread (or a sandwich) in the microwave too long and the bread gets soggy, slimy, and/or gummy? I think that's what's happening to dishes like General Tso's and sesame chicken, as both dishes (and other similar ones) involve meat being coated in flour/corn starch and then deep-fried (before being sauced). In a sense, these dishes are like bread-coated meat fried in oil and then sauced, and it's reheating that bread coating that makes them awful on reheat, like putting a sandwich in the microwave.
Mongolian beef and Kung Pao chicken are not coated in dough. Nor are they deep-fried. They reheat fine in a microwave. General Tso's and sesame chicken are coated and deep-fried, and they're always awful when reheated, in a microwave or otherwise. Reheating most fried food usually results in a soggy, slimy, gummy travesty, and I think (but don't know if) that's the issue here.