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.
Anyone know if Google Voice is encrypted? I can read copies of my texts online so I'm thinking no. I've felt like the service has outlived its usefulness for me and that would be the final straw.