Of course you can use the Star Alliance. Then you get all the uncertainties with alliance booking and the cancellation being discussed. If you fly with Avianca itself on its own metal, you'll get much fewer issues.

My intention is to fly Avianca. There are many other options with Star Alliance, such as Aeroplan, Singapore, and even Turkish airline.... If your flights get cancelled one or two weeks before departure, then you trip can be ruined. It takes months of planning for transcon trips. Why taking the risk to save some miles?