Soooo... My iMac had this same problem. Got an error on it every time I tried to wake it, saying that it had to restart. I also couldn't use the keyboard or mouse (both Logitech bluetooth and worked before) to wake the computer. Always had to use the physical power button in back.
Then, super early this morning, I woke up at 2 am to find my Mac on the grey boot screen. Okay, I thought. It's just rebooting itself. Again. Then, when I woke up at 4, same screen, no progress. Sat and watched it for another 5 minutes, just in case. Nothing. Hard shut down, then waited a few seconds and booted up. Super slow to load. 10-15 minutes later, I got fed up and installed the Combo Update for this version that everyone keeps mentioning. This was a much bigger file than the one initially installed by the App Store, but the Combo installed more quickly. The original took about 3-4 hours. So I set it in motion, then went back to bed.
When I got up this morning, I tapped the mouse button and it woke up the Mac. Woot! Maybe it's too soon, but I also haven't gotten any sleep wake errors either.
TLDR: If you're having an issue with this update, try the combo update. It worked for me, and I'm super thankful to all the people that mentioned it. Thank you!!