I base my opinion on Covert being better vs zoners on the fact that you will always have access to Military Stance, no matter the situation. With Demo, you usually try to use the grenades to get in, and upon doing so you end up having less ways of converting off a successful hit. Plus, Demo's need to reload can destroy her momentum, making it easier for zoners escape.
At the same time, it isn't like Covert has better tools for getting in, so you could argue that simply saving the grenades would make Demo better.