The problem comes when you already have Isofix in one car and you want to transfer the seat to other cars.
If the grandparents take the kids out then you may need 2 sets of seats and a lot of adjustment.
I would expect that the Isofix are better, in that there is less setting up during attachment. Less to go wrong.
The older seats attachments should be ok but they may not be fixed properly, especially if they are swapped from car to car. :nenau
I just looked on ebay, and there is a kit for the Ford Focus as an after market fitting.
But it would be unsafe and possibly illegal to fit this to other cars as there has been no crash test results, so don't even think about that method.
It would be a usefull attachment in a boat though, or even a stationary caravan.