If memory serves, SUL was developed for one purpose: to keep South American cops from shooting each other. The trainers that came up with it were instructing the SA officers on entries. In the stack, the officers tended to point the muzzles in every direction except away from their fellow officers. So, SUL was developed to guarantee the muzzles were down.
For those who like SUL, how does it help retention? I've gone through retention with it and the issue in this position, you don't have much leverage and your weapon/hands are already almost locked up anyway. If someone gets in grab range, they can push with one hand and lock both hands and the weapon against your body. The one officer in that case who used SUL did so only because it was SOP for his dept. and team.