Every fall when Apple releases new updates for their mobile operating systems, reviews like this one appear in the store:
This has been an annual tradition, and has happened each year with IOS 7, 8, 9, and 10, and will likely happen whenever Apple releases iOS 11 this year.
The first rule of administering Dynamics 365 Mobile is:
Do not let your users upgrade their mobile operating systems the first day that they are released
In every one of the past 4 iOS updates, Apple changed something that broke the Dynamics app, quickly followed by an update that fixed the issue. With iOS 10, they broke Office 365 authentication, and about a week later, released an update that fixed the issue.
So if you administer a Dynamics 365 deployment with a bunch of mobile users, either plan on downtime every fall or warn your users not to hit the update button as soon as they see the update is available.