Make sure the hydraulic brakes on the uhaul trailer work properly. I have had much success with the uhaul transporters but some people have claimed the brakes are not maintained or lack fluid.
Uhaul says all you need is the 2 tire ratchet straps at the front of the trailer. I guess it works. In general practice on flat bed trailers like that, cars are tied down in an "X" pattern. The trailer does have many additional tie downs to do it. That is how i tie down my offroad rock crawler. Plus straight down too. But then again it has a big lift and flexy suspension. None the less, you would want to pay attention to your review mirrors. If the car is bouncing around you may want to ratchet down some more. But the BMW is not lifted and on a flexy suspension either.
I'm not sure there even are 4 tie downs on the BMW. I only think i have the one screw in tow loop in the trunk, that goes into a hole either in the front or rear bumper. So in this case I would use the uhaul supplied straps at the front, and the tow loop in the rear just to plant the rear firmly down so it doesn't hop if you hit bumps. Maybe get another tow loop for the front as extra. Don't tie anything to your axles or drive shafts. Even though their safty chain might say tie to an axle. That only works for solid axles. Not IFS. And even there it's debatable.
Uhaul has a stated 55mph limit on the trailers. What's odd for me is that when i surpass that speed the trailer shakes. Maybe its just my tow rig or they have some speed limit warning system in the trailer. Others have reported this too.
Load the car forward engine first. You need the weight on the rear wheels of the tow vehicle for traction and braking.
In case you don't see the latches, The wheel wells of the trailer wheels fold down so you won't smack your car doors getting in and out.
Its a uhaul. I'm always a little skeptical. Check tire pressure in trailer and tow vehicle to make sure that you don't have under inflated tires, or driver side at like 40psi and a passenger side at like 26psi. Stop periodically, or whenever you do stop take an air pressure reading to see how the heat of driving and the weight brings tire pressure close to max tire pressure.
That's what i can think of now concerning towing with the uhaul transporters.