We had an isofix car seat in each car for our GD until fairly recently. Yes I know they can be moved from car to car, but we found it virtually impossible. Getting them out was easy; getting them back in, however, was an entirely different matter as the car's isofix fittings were so deeply buried it was extremely difficult to locate the seat fittings into them - and even when we did successfully marry them up, the amount of brute force needed to get them 'clicked' into place was beyond both of us. The only person who could do it was our son! Consequently we removed them only very rarely.
Now, thankfully, she uses one of the booster seats with back, secured by the car's seat belt.