According to GMG use of the downdraft will probably void your warranty.
According to Traeger, using any pellet but theirs will void the warranty.
I don't own a pit with a stack on it (mine has equally spaced vents on the back). Had I a pit with and exhaust stack on the side, it would have a downdraft installed. Everything I've read points to more even grill temps, better temp control, lower pellet usage, etc. I read that as 'easier' on the components that drive the smoker. Seems to me if GMG wanted to push the point, they'd have a hard time upholding their position given the abundance of info out there.
But that comes at a cost to the consumer. I really hate the bully stick that manufacturers drag out because they think they can.
That said, I'm not aware of either party litigating the matter.