S5 & S7 that I have doesn't play nice with some iphones
It's because iPhone uses a different type of SMS, It can check if the SMS has been opened like an MMS Read Report, Android uses the normal SMS rules that don't have read reports, only delivery reports.    

When the SMS is successfully received they're seeing that it's been successfully delivered, if you open the message, it'll say that it's been seen, like Facebook messenger in a way.    

The OS's use completely different methods of SMS and probably will not become compatible anytime soon unless Android finds a method to handle the changes.