I don't view SMS as reliable. It was only a couple years ago that I switched to Signal for a friend after our texts back and forth had been silently dropped one too many times.

I don't know how long RCS has been around, but my impression is most or all of my messages until recently were SMS.

iMessage now says "Text Message - RCS" or "Text Message - SMS" in the text entry box which is better than the green/blue bubble thing (though it does still have that).

At a minimum, it isn't encrypted between iOS and Android users.