Oh, and the problem wasn’t actually the plastic intake manifold itself, but the coolant crossover (molded as part of the manifold) which had a long seam in the plastic. It wouldn’t seep or leak as much as BURST.
The replacement model is a plastic intake manifold with an aluminum coolant crossover subassembly.
Yup, I was being oversimplistic - one of my relatives with a Mustang got bit by this problem, sued Ford, and won.