Zero risk. I haven't faced any issues on my lumia 920 and 520. I am enrolled in dev preview program since GDR3 was announced. There are many users having issues with new OS , but it's still a preview not the final build.
I would disagree that there are 'Zero Risks', which, btw you contradict in the very next sentence.
If you consider, possibly not being able to use Wi-Fi (if you do, you risk the power button bug), there are some having serious battery drain issues, with various other instability issues all noted in this forum.
If you're not using a Nokia device, you can't go back to 8.0 if you don't like it, or have a more serious bug. That is a definite risk that needs to be acknowledged.
Will the update succeed, likely yes, just know that its not perfect (but very stable), as any preview software often is...go in with your eyes wide open, and you should be pleased overall.