That the spring on the car in question cracked refutes the idea that composite springs do not "wear out". They don't sag or change rate as do steel springs but they can break or crack and that qualifies as "wearing out".
IMO, there is no significant change in ride with a plastic spring, though there is an incremental change due to the the lack of damping caused by inter-leaf friction. Because of that modest decrease in damping, more aggressive rear shocks are advised, but are not absolutely necessary, when a steel spring is swapped for a plastic spring.
When doing the opposite, you may notice the rear end seems a little more stiff.