iMessage does not, and never has to my knowledge, fallen back onto sending text or picture messages when your data signal is low. You need to select "send as text message" if it fails to deliver using data. As long that remains the same with RCS there is no problem.
If you select “send as SMS” option on Messages settings, then it will send as SMS when iMessage is not available. I have this enabled on my iPhone, this is why I get impression of iPhone default to SMS.
For Google’s message, when RCS is not available, it will default back to SMS, but it will tell you that the message was sent through SMS.