There's really no easy answer to that one, as the reason for the incompatibility varies on a case by case basis. Here are some on the common reasons for why Radians *can* have problems.
Not all cars have latch, even the ones who do have it, do not generally have latch anchors strong enough for a child to continue using the latch instillation past 40-48 lbs.
Not every car has latch in the position that the car seat needs to go into.
A lot of times, latch does not allow the seat to be installed as close to the door as the seatbelt will, so parents needed to put two seats next to each other may have to use the seatbelt even though the cs is in a seating position that does have latch.
Contours of vehicle seats vary, as does the placement of seat belts. Just because the seatbelt locks, it doesn't mean it can tighten enough around the seat to install it correctly without an extreme amount of work. If a seat takes 2 techs, and a really big fireman to install it correctly, the parent will never be able to recreate it on their own. The round peg may be able to be forced into a square hole, but unless the parents are going to carry all the people needed to do it around in the trunk, it is better to just pick a seat that doesn't cause so much hassle.
Latch and lockability standards were intended to cure car seat incompatibility issues, unfortunately, they are about as successful at making all car seats fit all cars as shoestrings are at making all size 8 tennis shoes fit all size 8 feet. There is just way too many variables, and unfortunately, the features that make seat belts safer for humans, make them harder to use for car seats.
Personally, I have not had the Radian not fit in any of the cars I have needed to use it in. But I am not your average parent either. I have a few (well, a lot of) tricks up my sleeve to help me out. Plus, I'm WAY more stubborn than most
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Kimberly