For slides I get better results shooting them with my camera than using an Epson V600 flatbed scanner. I don't know if that would be the case if using one of the better scanners. There are two main advantages. The first is speed. I can go lots faster putting the slide in the adapter, verifying focus, and tripping the shutter with a remote release than I can getting slides into the scanner and waiting for Vuescan to do its thing.
The second advantage is dust. The scanner is a dust magnet. I have much less of an issue with dust using the camera. I wipe a box worth of slides down with a Pex Pad before shooting. I hit both sides of the slide with a rocket blower before inserting it into the adapter.
Down side is focus. The cheap Canon lens changes focus when you breath on it because of the weight of the adapter. You also have to jump though a minor hoop to shoot at other than wide open. The DOF wide open with the extension tube is maybe 0.5mm! I shoot at f/5.6 or f/8 -- don't remember exactly which one. Manual exposure with the meter reading 1 stop over seems to give me the best results.
Example: My brother on his way back from Baja in '73.