All peel apart instant products stop developing when you peel them apart. They have an acid layer and a timing layer to reduce alkalinity.
All integral instant products have a timing layer that kicks in roughly 2 - 5 minutes after the pod bursts and this brings the pH down which shuts off development.
All instant products, just like all normal photo products are temperature sensitive and the results vary with temperature.
So, the answer is "NO" they do not develop to completion, but they can suffer from temperature effects which can over or under-develop them.