For some reason, the feature is enabled by default in iOS 9, and it isn’t easy to understand when it is active (in fact, the only explicit sign is that the Wi-Fi logo on the phone’s screen becomes greyed out). This is why users have reported accidentally running up large bills for mobile data usage because they believed they were on their home Wi-Fi.
Recently, a class-action lawsuit was launched by a couple of users who received an overage bill. The customers charge Apple with negligent misrepresentation and with breaching Unfair Competition Law and False Advertising Law, and want the company to reimburse affected users, alleging that the total potential damages could exceed $5 million, taking into account that Wi-Fi Assist is enabled on every iOS 9 device.
Apple was quick to update its website to clarify that users “may use more cellular data” with Wi-Fi Assist on, while pointing out that this should only be a small percentage higher than previous usage. However, the plaintiffs do not agree that this is enough, pointing out that average consumers use their iPhones and iPads for streaming content, which can use significant data and thus lead to huge mobile data bills.
It should be noted that Apple is not the first company shipping such feature to its customers. For example, Samsung, HTC and LG also have equivalent features on their own phones, but none of the Android manufacturers have faced a similar lawsuit. Yet.