I found Google messages to be unreliable: refusing to send a SMS if the Internet connection is bad. The signal that the message failed to send is a single hollow checkmark.
I switched to fossify messages, which just sends SMSs or MMSs and doesn't create its own flawed messaging protocol
SMS and MMS are not very secure however. RCS is technically an open standard. it's mostly controlled by Google at the moment, but hopefully that will change as Apple enters the game this year.
If I want to have security, I would use a different communication protocol. I find it unacceptable for an SMS app to change quietly change to a different protocol, particularly if it causes messages to fail to send.