The issue is how the data is backed up on Android vs iOS. In theory, if your iCloud or Google account is the same between devices then transfer to a new, same-OS device is trivial. It should "just work" in most cases. But WhatsApp does not provide its own backup service (which is fine with me) nor does it allow you to specify where to back it up to (which is not fine with me). If it did, then users switching between iOS and Android would have no (or little) trouble.