tldr; neither safe nor unsafe; operator forethought and skill will be key.
I would think a good fit in a divided leg/U sling will allow your legs to take more of the pressure off your torso in the sling, to be more upright, which in turn helps you keep a more neutral spine. Slings have different levels of back support, and you can ask the PT or whoever manages your osteoporosis what they think, but I would imagine you would not want the kind with a plastic insert because of pressure. And if you use adaptive clothing you will want fasteners in front. I would never use an extra back cushion anywhere as you need the form-fitting "bucket."
I would also suppose that you would not want to go any distance in the Hoyer (and if you do have to bc of space issues in the room, have someone stabilize your legs/back as you roll esp. if there is carpet) but rather do everything in the same room if possible. I think the #1 rule would be not to rush and always to get a neutral spine position before moving the lift or transferring you.
Preparing the wheelchair, lift chair, or bed at the right height/angle to receive you from the sling and immediately adjusting further as the transfer concludes, can reduce any stress/pressure on your spine. I would certainly ask the PT/OT/whoever manages your osteoporosis/spine to weigh in on any other part of the procedure that can help, send pics of your position in the sling, etc. Maybe make a video showing a transfer and see what your team thinks.