There are some clear warnings. You will need an up-to-date beta of Android Messages (5.2 as of this writing), an application that can view Android activities (such as activity launcher) and WiFi. And this is the assumption that the function remains on. Google may opt out of this decision with a little warning.
Despite these limitations, people have succeeded in using this method on different devices, and the four major US carriers and at least some international networks. It is then technically possible to make widespread RCS. With that said, allowing universal RCS access may not be just a matter of switching a virtual switch. Google's official approach involves users' customers pinging each other to see if messages from the next gene are available. He may not want to deploy the ability beyond France and the UK until he knows that he can easily and reliably push the function of everyone.