Assuming the wheel spacers are hubcentric with both an internal counterbore to fit the BMW hub boss and an equivalent boss on the spacer to hang the wheel, they should be safe, as long as they have the right chamfers on them. As I've mentioned before, an F87 lost a wheel at a track day when the wheel stud kit studs sheared off after only a few hours of use, due to incorrect chamfering of the wheel spacer. This video explains: