I've had Band 1 and Band 2 pared with my iPhone (6+) for over 1.5 yrs.
It does pretty much everything its supposed to do, except Cortana integration with iOS. No keyboard. Frankly, I don't care about Cortana and Band working together so never really explored it. For all I know, it may work together but I don't have Cortana installed on my iPhone.
Notification, calendar and reminder, caller ID, messages all work just as advertised. Obviously, all the health tracking is the same and there is MS Health app on iOS. I also use app called Band Health Bridge to integrate Band activity data into Apple health app so that I can see a single page of data from different sources (smart scale, blood pressure monitor, myfitnesspal data).
So, it would be inaccurate to simply say that some features are limited without saying more about how it does work. I'd say that vast majority of the features works just fine. There are some things not supported, mostly due to iOS restrictions.