The only two features I can think of that iMessage has which discord/whatsapp/etc don't are:
1. Automatically can flip to "do not disturb" mode when my phone is in sleep mode, and display that notice to my companion. Other chat apps require manually configuring that I think. I also suspect apple doesn't provide an API to make this easily possible for other apps.
2. iMessage makes it easy to identify the poors, the android users, and ignore them. If I exchange numbers on a dating app and get a green bubble, I can know to immediately ghost them.
Are those the features you were thinking about, or something else?