The figure 8 is fine for confident cats accustomed to being on a lead
outdoors. The problem is, most cats are not confident for quite some
time and will spook easily outside. The first thing they do is pull. The
next thing they do is go backwards and spin when they hit the end of the
lead. This motion is almost guaranteed to spin them right out of the
figure 8, like when you shrug off your tee-shirt right over your
shoulders by tugging from the back.

Cats have no collar bones. You (that is, a general you, including the
manufacturers of these things) cannot rely on their shoulders to keep
them in a harness. A figure 8 slips off over the shoulders as a cat
backs up, like a human's tee-shirt as they pull from the back over their
head. There is no separate collar, so the whole thing comes off.
The H pulls in the same direction but because of the buckles on the
bottom and the separation of the chest and neck straps, it puts pressure
under the armpits, helping anchor it. If they manage to wiggle out of
the chest strap, the collar is attached separately so it is still on the
cat.