In app browsers are designed to lock the user into the app they are currently using, allowing it to track the user after they have followed a link, and coerce them to come back. For the websites being visited (on iOS) this is a separate session, no shared cookies with the main browser. This fundamentally breaks shopping carts on online stores, and I have seen it result in a 50% drop out rate at checkout (users often don't have access to their stored card details).
I frankly can't understand why there has not been more push back from advertisers using platforms such as Facebook, they are only a net negative for them.