Messages between two Apple devices are safe, and messages between two Android devices are safe, but messages between an Apple device and and Android device are vulnerable.
This is not very accurate. Some Android devices come with Google Messages, which will use Google’s encrypted version of RCS if the carrier supports it. People who don’t know what all of that means should not assume their messages are encrypted.
I see no reason to believe that it is.
I find it useful when outside the USA to be able to communicate with American luddites who refuse to install messaging apps.