I’ve used the iMessage channel a couple times, seems to work fine. The one thing I’m kinda annoyed about is that at the beginning of the conversation it asks you if you want to sign in to your SkyMiles account. If so it gives you a time to tap that opens a popover to sign in, but the first time I did it I was writing about a relative’s reservation so used their number. Since then, even weeks later, tapping to open the login automatically logs me back into the same account with no option to change it.