I'd say it is a "risk" to wait for the new phones, though it isn't likely much of one -- other than you won't have a new phone for about 4 months (Nexus typically are shipped end of October/beginning of November). The risk is that the new Nexus phones may not be as "good," say more like a Nexus 6 than a 6P and that they still may not be fully Daydream capable. We likely won't really know this until, at a minimum, at the Nexus launch event, and even possibly when the first units are reviewed in October.
Now I put "risk" in quotes because likely you'll still be able to buy the 6P in October/November, and likely at discounted rates -- though likely not directly from Google or Project Fi. But, again, you'll have waited an additional 3-4 months before getting your new phone.