When is digging better than a draw engine?

Maybe this question isn't asked very often because a draw engine is basically just digging through your deck as well but what I mean is, a lot of draw engines go through your deck one card at a time while rummage/loot/mill effects will work 2-3 cards at a time maybe more. So i guess my question is, in the non obvious situations such as when you have synergies with the rummage/loot/mill effects, which would you use and why.