I've spent the last 6 day in xCode 8 trying to layout storyboards and for 6 days have had issues with constraint and the storyboard preview not displaying anythng correctly. Constraint errors up the wazoo. I have also read through the forums here and on stack overflow and noticed a lot of folks having the same problems I have had. So just for the heck of it I downloaded the xCode 8.1 beta and it looks like some of the issues have been fixed. So, for all those who have posted issues here you should do yourself a favor and check out 8.1. I found it to be somewhat of a steep learning curve but once I figured it out and started using 8.1 my layouts have been working, have had no errors, the preview in the assistant editor and storyboard all display the views properly for all device variations. 6 days in xcode 8 and no luck, 1 hour in 8.1 and Im happy again. I did have to change my approach on a few things though.
Good luck.