I kind a understand where you are coming from and Previews are definitely a nice addition to test what is coming up. But at the same time, I wouldn’t expect a subsequent Preview to be stabler as the previous, because I always considered them a special branch of allowing the community using the binary version to work with the developers directly, since that was only possible in a direct manner on the Source Code Branches.
I would expect that Epic tries to clarify a bit clearer and more to people what Preview means and even put up a red button in the Launcher. I was a lot on Answerhub during the time and a lot of people broke even Live Projects with no backups, completely not understandable since it was saying Preview, but than, different people with different backgrounds understand things a bit differently of course.
Allowing Epic to bring forth Previews to the community and those who want to engage, to collaborate to make a better Engine and take sometimes risky steps, for the diversity of use cases, is a huge thing and I suspect, no I am pretty sure that with a stable release, Epic wouldn’t release anything which is known to them breaking something.
Secondly, this being a very rolling release approach, next of using an open community approach, with handling two different streams (source and binary, generalizing here of course) this is more than I could ever want to be feel to be part of helping and creating a better Engine with those in charge.