Slow cookers are designed to cook for long times at lower (below boiling point) temperatures just as sous vides are. The superiority of sous vides over slow cookers lies in much better temperature control. the water bath buffers out variations in temperature that invariably occur in slow cookers due to their design. A slow cooker would not reach boiling temperatures unless it was defective.
the most significant difference between cooking in a slow cooker and a sous vide is at the food in a slow cooker is in one solid volume. Was sous vide’s it’s better to not have the container too thick so that heat can disperse better. just using multiple bags would be an effective solution for that. that’s more of a hassle than just dumping the entire volume in a crockpot. But the vastly superior temperature control of sous vide’s plus the elimination of any possibility of evaporation have been extensively proven to give vastly superior results.