If you’ve been blowing through data since upgrading to iOS 9 or buying a new iPhone, it’s probably not because of Wi-Fi Assist; it’s because of your bad settings.
Wi-Fi assist is a new feature Apple introduced in iOS 9. Enabled by default, the feature helps ensure you are always using the best connection possible. So if your Wi-Fi signal becomes weak or unreliable, the setting will automatically switch you over to your cellular connection.
iOS 9, and Wi-Fi assist, have only been out for two weeks but many have already been quick to sound the alarm bells that this setting will chew through your data and cause those dreaded overages. But this “issue” is massively overblown. Yes, it’s true WI-Fi Assist will prioritize your data connection over Wi-Fi in some situations. But the reality is it’s extremely unlikely that this setting is what’s causing you to blow through your data cap.