seat belt fitment
It is perfectly safe to use a seat belt fit child seat, and when fitted correctly, there is little difference in safety performance to an ISOFIX seat.
However, belt fit child seats have more room for mistakes to be made, which is why ISOFIX is generally recommended.
They also don't need a vehicle compatibility list like ISOFIX seats with a support leg (unless it's a belt fit seat with a support leg!), so it's difficult to know if the seat is compatible without getting professional help.
There are instances when the set up of the vehicle seat, seat belt and buckles do not work with some belt fit child seats, causing an incompatible fit - in a crash, the seat is very unlikely to work correctly.
One problem that can arise is buckle crunch.
This occurs when the adult seat belt buckle is pressured at an angle on the child seat - if you fit the child seat and the vehicle buckle is sitting over the frame of the child seat, it could fail in a crash. This is because the child seat itself will come forward, putting pressure on the buckle in a way it isn't tested to endure.
If you have buckle crunch when you fit your child seat, try it on the other side of the car as buckle lengths do vary even within the same vehicle!
If you still have buckle crunch and the child seat doesn't have an alternative belt routing, then it is not compatible with the car.
Forward Anchor Point
This occurs when the vehicle seat belt anchorage point is forward of the seat belt route for the child seat.
When fitting a seat with a seat belt, the belt must pull the child seat back into the vehicle seat.
If any of the points where the seat belt is attached to the vehicle, is too far forward of where the seat belt goes on the child seat, it cannot pull it back tightly, and the seat will wobble.
In a crash, the seat will move more than it should, which puts the child at increased risk of hitting the inside of the car. The seat and seat belt are also less able to absorb crash forces away from the child.