Besides all the other reasons already mentioned, its security model is somewhat dated. For example, it does not use forward secrecy, neither for chats nor for 1:1 calls. (Group calls do have forward secrecy.)
That said, I still use it over Signal when I can – cross-device messaging is very hard, UX wise (which device receives the notification, does "mark as read" properly sync across device, how is the behavior of a device coming back online after a while etc.), and Apple has almost perfected it at this point.
That said, I still use it over Signal when I can – cross-device messaging is very hard, UX wise (which device receives the notification, does "mark as read" properly sync across device, how is the behavior of a device coming back online after a while etc.), and Apple has almost perfected it at this point.