Indeed. It's difficult to see why a childminder wouldn't be a preferred option in this case.
Not quite attachment related, but I was part of an early years project which produced significant qualitative data about the impact of grandparent care. The gist of it was that children coming from grandparent's care to school settings were socialised to be less physically competent and mature (i.e. less outdoor play, less exploratory play, less messy play etc due to either opinions or physical limits of carers). This affected their current level of development, and could be a predictive factor in future development.
Now, this anecdata wasn't taken further as far as I know (no appetite for it!) - but it is an example of how little understood and how contextual child development studies are. There's no way you can anticipate all of the possible influences on a baby and predict the unpredictable. You can only look at the baby in front of you and decide what works best for you and them.
For our son, we were happy with nursery, because whenever we have taken him out and about, he's always wanted to hare off by himself. Our nursery has a huge outdoor space dedicated and free access for under twos. He only seems to want an "attachment" as such every so often, and is also task-oriented - so he follows around whichever nursery staff member is doing chores and copies/"helps".
Incidentally part of the reason we don't want my MIL to do weekly childcare for our son is because she doesn't let him play normally. She's always trying to get him to sit down, and sit still, and stop him doing perfectly safe activities.