This image was taken by my 950XL of my old Lumia 640 when I added it to the insider yesterday (for some reason taking a screenshot directly on the 640 was failing). I'm
assuming it's the same on the 950/XL. "In some circumstances, you may not be able to place calls (including calls for emergency services)."
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And you're right, people read things here, or hear it elsewhere (I believe I've even heard Paul Thurrott say people should be on the insider program to get the latest builds) and join to install preview builds. Unless you pay attention to/know what you're doing, you really don't know what is or isn't a preview release.
I would say you should install preview builds if a) you're willing to accept the risks, or b) MS has released a build to the carriers or is pushing it out to unlocked phones. I think once a build goes to the carriers or is being pushed to unlocked devices, it's not beta. But then again, you could argue that if MS pushes a new build out to the unlocked 950XL, but it's not being pushed to the 640, then it's still preview for the 640 and not suitable for a 640, 950, etc. All this to say: it really is caveat emptor. If someone isn't sure precisely what it means and what the risks are, they probably should wait until their phone tells them there's an update.