This is one of the main reasons I am switching back as well. I've kept my 6 plus while mostly using the Note 4, S6, G4, and Nexus 6 the last several months but had not really used it until the iOS 9 beta was released. Really forgot how much I enjoyed iOS and it is even better now with iOS 9. iMessage is the thing I missed the most and you don't really realize how bad messaging is on Android until you come back and use it. Hangouts is definitely on the right track and is as good as, if not more powerful than iMessage, however with even more people adopting Android Hangouts has failed to gain any traction. Most people just use Samsung's junk message app or whatever the stock one is on the device they purchased. Email is another sore point on Android for me. Obviously it is fine for gmail, but the iOS email app is far superior to any solution Android has for email accounts other than Gmail.
Another small (or maybe not) reason I am jumping back to iOS is to talk myself into getting an Apple Watch
Liked my Moto 360 but Android Wear is really not user friendly at all in my opinion. Played with the Apple Watch a few times at the Apple store and if I can overcome my fear of the severe tail kicking my wife will give me for purchasing it I will have one in my possession soon.