I too, as a c/m do a lot of shared care with grandparents, some of the gp's like it to begin with, but when the realisation hits them, that this is for every week, 50 or so weeks of the year, it is a tie, and can begin to find it a chore. They can't have a lie in, go out to lunch with their friends etc as they have to manage their diary aaround the child.
They all started off with good intentions, wanting to see more of the grandchildren, help out with childcare costs etc, but it does have other 'costs' involved.
I have seen the parent get annoyed at the way things are done, not the way the parent wants, etc also when the gp decides they want a few days off, there is chaos and resentment on both sides.
I have 2 families this week looking for extra days as gp's can't cope anymore, and want to stop.
It can work, but it needs a lot of thought and give and take on both sides.