Good question. It depends on the depth of the scar. Autologous fat is placed in the subcutaneous layer - below the skin - so can be used for larger scars with loss of subcu fat.
Hyaluronic acid fillers like Restylane, Juvederm Ultra work best within the skin itself - so for shallower scars. Juvederm Voluma works well for mid depth scars and deeper scars and has the advantage of lasting longer than Juvederm Ultra.
I prefer combining Voluma and Juvederm Ultra as we don't perform fat transfer in our practice and we obtain good results with this combination. Considering the added cost of the liposuction and processing required for fat transfer most patients are not treated with autologous fat for acne scars.