Their first excuse was the controller was bad.
For $1700 you don't need any excuses, you need reasons. You might have just found your issue -- excuses! Take it somewhere else and get it done right.
If you feel comfortable jack up each wheel and turn it by hand while somebody is manually activating the brake controller. You should hear a slight drag before activation, then you should hear the magnet activated and the wheel stop. If you know your way around a volt/ohm meter check the power at each brake. Others have said to drive some miles with the brakes activated, try that as well. A football field size is insufficeint to accomplish that. But, first check the voltage at each magnet and make sure the wire is not broken. Then check to make sure each magnet is activated and the shoes correctly adjusted (a slight drag when turned by hand).
Many of us carry an infrared temp probe (inexpensive at Harbor Freight and other places) and after driving five miles with the brakes activated check to see how hot each brake drum is, each should be about the same. If one or two is/are significantly cooler than the others, you know the first place to look.