...flex cuffs & zip ties differ.
Functionally, they are the same, but the material of construction differs.
Zip ties will simply break with sufficient linear force, and even easier with angular force (twisting).
The very purpose of flex cuffs makes that "flaw" of zip ties, a problem.
Flex cuffs very likely have a substantial amount of a urethane elastomer that increases flexibility, strain resistance, and typically increases tensile strength.
So, lots of people use zip ties, but flex cuffs, by the chemistry used to make them, are designed for one purpose.
We don't need those, as folks just living their daily lives