iTunes Connect is the official “payment system” behind an iOS app. You can set up custom events in Facebook, but when you have a product that runs on a subscription model, iTunes Connect will have the info on repeat purchases / renewals / cancels. And Facebook won’t. So in the long run, you’ll lose valuable info and can’t measure real LTV for a certain user. Or for certain campaigns. Also, Growth Accounting and Growth Modeling can become problematic in this case. If you only have single, one time in-app purchases that do not renew, the Facebook event tracking might make sense, but here it was a different use case.