The common concerns are neuropathic. The first is about the autonomic nervous system; the tightening of blood vessels to stop passing out. Some people with diabetes experience light-headedness resulting from autonomic neuropathy, but movements can be safely executed while the lifter stays mindful of how they are feeling.
The second surrounds retinal detachment. The data shows the fear to be unwarranted by the number of cases. There are few known incidents of retinal detachment in gyms and far fewer where the victim had diabetes.