Motorcycle carriers (and load platforms) are not recommended for the X5. The Expedition referenced above has a full frame for the hitch to attach to. The frame can certainly take the load. However, the X5 is a unibody, with no frame. The OEM hitch is the best of the options available, and it comes with published figures for load limits. For the E53, the load limits for the OEM hitch are:

Maximum towing weight without brakes: 1650 lbs
Maximum tongue weight for trailers without brakes: 330 lbs

Maximum towing weight for trailers with brakes (off road): 3300 lbs
Maximum tongue weight for trailers with brakes (off road): 330 lbs

Maximum towing weight for trailers with brakes (on road): 6000 lbs
Maximum towing weight for 3.0 with Steptronic (on road): 5000 lbs
Maximum tongue weigiht for trailers with brakes (on road): 600 lbs

Maximum horizontal distance (measured from receiver pin to centre of hitch ball): 8"
Maximum vertical distance (measured from top of receiver to base of hitch ball): 5.5"

The issue is that the load creates a bending moment on the hitch receiver. The on and off road differences are due to an expectation of smooth roads or not; the off road limit is due to the trailer bouncing on the hitch, similar to a load carrier when you go over a bump.

If you assume you are not applying any dynamic loads, ie, applying the brakes while driving, then you can carry 600 lbs, but you need to have it within 8" of the receiver pin, with the centre of gravity of the combined bike/carrier unit no more than 5.5" from the receiver. If it is 16" out from the receiver pin , not unreasonable for a motorcycle carrier, then you are reduced to half the weight, or 300 lbs (including the carrier). If you have a higher centre of gravity, you need to factor that 300 lbs again, for a net safe load of x% of 300 lbs. If you want to consider braking forces, then cut the load in half again. Should cover a pocket bike, but not much more.

If you are using a non-OEM hitch, then you are that much worse off for potential damage to the rear unibody.

Get a single rail motorcycle trailer. It will tow easily, and will save your X5.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue