Not sure what parent was referring to, but for me iMessage frequently gets confused when dealing with devices that don't have a phone number or when moving SIMs between phones. Sometimes messages silently get routed to other devices or seem to disappear into the void in these cases which can be extremely frustrating. Since it's Apple, there's no way to tell when this is happening and try to correct it either. In theory iMessages tells me I'm sending a message to John Smith, but am I sending it to [email protected] or 555-122-4455 or some other alias associated with that Apple user? No way to tell until you get some error in iMessage that the message could not be delivered and no way to explicitly select which alias is going to receive it as far as I can tell.

Edit: Admittedly, I have a poor mental model of what is even happening in these cases. Are these "aliases" or something else? Unfortunately, Apple doesn't help elucidate the problems/mechanisms here. The only insights I have into what's happening is from mysterious errors and failure to deliver messages.