re: Pork roast and rice and gravy

There are a few downsides to using cornstarch in a gravy, though I have been known to do it in a pinch. It will start to break down if you are simmering for a while and stirring often. The liquid also needs to be literally just under boiling or the cornstarch will not activate. Corn starch will not stand up well in the fridge, if you have leftovers. The gravy tends to get watery left overnight in the fridge. This happens less in flour based thickening. In my experience, corn starch tends to muddle or dampen flavors a bit as well. That may be a personal thing though.

There is room for both in the culinary world. It all depends on the flavor profile you are going for. The rich nuttiness of toasted flour cannot be achieved with cornstarch. The clear silkiness of a gravy thickened with cornstarch cannot be achieved with flour.