1. It's free
2. It's preinstalled
3. It's well integrated
I don't think I'd use Mail, iMessage, Safari or Preview if it was not installed by default and set up so it is a default app. Feature-wise I'd say Apple software it good enough not to bother installing anything else and that's it.
Group Message often not delivered.
Group members often get drop out of Group messages for no reason. They thought no one was talking when that is not the case.
Their message to group will then appear to each individual member.
To solve this problem we have to restart a new group.
iMessage for years dont have searching. ( It is a bit like App Store where they just didn't bother to improve its searching )
And this isn't old. As recent as 2018 that was still the case. 7 years after its first introduction. We tried it as group every year and every year we felt disappointed.
WhatsApp on the other hand has been near perfect. ( Apart from being part of Facebook )
Not in US. And specially not within teens in US. ( And you can exclude Japan, South Korea, Taiwan and possibly Australia as well. Edit: And of course China. )
To this day I still dont understand how anyone use iMessages on a day to day basis. It is possibly the worst form of instant messaging I have had for years. Even worst than ICQ.