ISOFIX is no safer than the traditional system, ISOFIX is just easier to get right.

I had good training when the seat was installed at the ambos. A correctly fitted seat does not move at all. I have seen some horribly loose seats in cars.
Cars with locking belts make it easy to lock the seat into place.

It is interesting to know that ISOFIX was invented by Britax and VW. The 1st car to get it was the MK4 Golf.