The reason a lot of people don't like to have the subs downfire is there afraid that over time the cone will begin to sag down over time. Some subs are designed so that this will not happen but many large drivers are not.
How much the cone sags depends on how heavy the cone is, all will sag some amount. However much it sags directly subtracts from your useable xmax, that's why they are "less desired." The rule of thumb is to call it okay if sag is less than 5%. They may sag more over time if the suspension on the driver loosens up.
Percent Sag = (Sag / Xmax) * 100
Percentage of Sag = 24,849 / ( Xmax * Fs²)
Percentage of Sag = 981,000 / (Xmax * (2 * Pi * Fs)²)
For my Tower Sub project in my sig, sag is 2.64% (0.33mm).