White sauce without roux Recently I have been making white sauce by:


heating the milk in a pan until nearly boiling
mixing corn flour with a small amount of cold milk
removing milk from stove
add corn flour mix to hot milk 
stir until thick.


This is very fast and never produces lumps, but what are the disadvantages versus using the traditional roux approach?
If it works for you (you like the results), there really isn't a disadvantage unless you have leftovers. Cornstarch (cornflour) thickened sauces tend to thin upon reheating more than roux thickened sauces. Depending upon what you are making, you may or may not notice it much if at all.