The never ending joke on set is fix it in post. If you’re not fixing it in preproduction and have a solution that’s been tested, then you are going to spend more time in post for less results. Too big of a proportion of my work load is from footage that was shot without a visual effects plan. Or the plan is, yes visual effects there please.
Being an on set vfx supervisor is a lot of standing around unless it’s a real visual effects heavy project. The shots you are needed for are usually sandwiched in between a lot of other shots needed on that schedule. Also you are the voice of a lot of retakes when you are needed. So I can see when the people picking up the bill don’t want to raise the budget for it. A good vfx supervisor will catch problems that while equal much higher than their wages.
When there is not a vfx supervisor on set usually someone braves stepping into the role and coming up with solutions on the spot. I’ve had an editor tell me there was no supervisor onset so he stepped in as one, but did state and restate he was not a vfx supervisor. His desire to achieve in the role was a little too eager. There were massive bounding boxes in gaffer tape around all the markers on gun shot wound victims and television screen replacements. It was an extra day or two in total too cover up all this and it’s directly where people are going to be looking.
If you are hiring visual effects work after you got your shot it could easily effect quality/cost negatively. The input in preproduction and production will allow a vfx artist to execute a plan instead of figure out solutions. It won’t eliminate problems but will reduce them enough that the artist can spend time on perfecting the shot.