I bought an Isofix base for use with a baby carrier and used it with no problems for my first child. When using it with my second child, the mechanism attaching it to the isofix link thing malfunctioned, meaning the base couldn't be removed from the car. I had to get a mechanic to cut it out of the car. Just outside the warrenty of course.
Saying that, it's better to have it jam in the safe position rather than the unsecured position.
Modern car seats are huge. You can't fit three of them in the back of an average car. One seat for a toddler takes up more space than an adult.