Hangouts has SMS capabilities as well.. in fact.. the Hangouts app is feature rich is the main problem.. it tries to do everything.
Anyway.. tested Duo out when I went away for work recently and the experience was as good as FaceTime if not better.
Additionally, Allo has been great as well.. I only use it with the wife mind you.
I would prefer SMS capability on Allo but not a deal breaker as I rarely SMS anyway.
Just need Google to tweak the SMS capability on Allo and push/promote the heck out of both Allo and Duo.. then I think we will have plausible alternatives for iMessage and FaceTime.
They both already have a cross platform advantage!