The low profile tires (keeps the X5 lateral motion in check) and relatively short distance from the hitch ball to the rear axle (short lever arm) when compared to most other tow vehicles (Surburban, pick-up trucks, etc.) mitigates sway by not allowing the trailer to "wag the dog". The transmission's manual mode when used with cruise control allows max throttle without "working" the transmission.

I don't believe BMW realizes the capabilities of their vehicle, just as other manufacturers don't really understand towing, but they seem to have stumbled into creating a really excellent tow vehicle when configured correctly (i.e. sport package, air suspension).