I've been towing with unibody vehicles for decades without issues, as have many others. There are unibody vehicles that can tow up to 7,700 lbs available (VW Touareg). The "unibody is no good for towing" is a myth, if one stays within all manufacturer recommended limits.
On the Q5, as others have mentioned, the most likely limit that would come into effect is payload capacity.
Also keep in mind it's a fairly short wheelbase, high center of gravity, vehicle so you'll also be better off using one of the better WD hitches with sway control.