You haven't provided any information about the spacers. They can have some really nice advantages, but they can be bad as well. Usually, the wider they are, the more their negative effects will be magnified.
In general, even though you have put spacers only on the back, your steering may become a little heavier, due to the wider turn at the rear. The car will also understeer, requiring more aggressive turning, loading your steering system, front tires and suspension accordingly. Since the leverage on your rear suspension is now magnified, expect faster wear on all four corners of your suspension. Of course, your suspension is not tuned for a wider track, so the vehicle is unlikely to be as stable as before. Often this is not a concern to the driver who installs wheel spacers.
Obviously, clearance between the rear tires and body is reduced on the outer edge of the tire. Bottoming out after riding into dips or over humps may actually create interference between these components with the potential for damage and the reality of accelerated wear. The added strain to your wheel studs will add to the risk of cracking them, particularly upon impact with road debris. If your spacers are cast aluminium, instead of forged or machined billet, then they may crack easier, the holes may become out of round and you may develop "mysterious" vibrations arising in time. One common consequence of inserting spacers is the reduction of wheel bearing life.
It's important to check the terms of your insurance coverage. Wheel spacers may invalidate cover in the case of you being responsible for an accident.
Tens of thousands of owners have little to no intolerable problems. I would hope you will be one of that fortunate number.
To address your question, you appear to be worried that your new spacers do not have a lip to locate the wheel on.
If that is the case, I would buy some new hubcentric spacers, preferably ones that bolt to the car, then your wheels bolt to the spacer. These are generally available in size 12mm upwards.
You want the type that look like the following:
If you are using the latter type, please ensure the wheel bolts have enough engagement in the threads of your hub (minimum 1.5x diameter).
I've not got my XF yet so I don't know if the car has bolts or nuts.
But ultimately yes, the stress/load is now on the studs if your spacers don't have a lip to mount the wheel centre on.
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