Currently iMessages are causing considerable problems with silent fails and no option to send as SMS (which they used to have - not sure why that changed). All the other messaging solutions have some fairly big flaws too.
Also: i really really fucking hate the way I'll get a notification with a partial message, and then have to open the app to read the message. Opening the app to read the message has a delay of maybe a minute before the message downloads - why? Why can they show me a preview in a notification and then take an age to give me that same text in their app? This is particularly bad with Facebook's messenger app (so bad I'm tempted to delete the app and leave different contact details on FB) but als happens on Whatsapp and others.