For me, I can usually forecast the mixing work pretty well if I've got a good set of stems. Unfortunately, it's editing that can blow the schedule all to kingdom come. If it's a couple of pops or clicks, no problem. But if it needs comping, tuning, vocal cleanup (breaths & noise), tightening up drums and bass (dynamic or manual splitting and quantizing), I have to make a decision to either do it myself or send it back. It seems most mix engineers have a price point for editing ($25 to $250 an hour depending on complexity), I'd just rather not do it anymore unless it's my own mix. I used to enjoy it, but now it's just tedious and can add hours or days to a project...
