Nova works great with knock knock as long as you knock on the notification bar and not on the screen. Double knock the center to wake it up works great and it works every time. Turning it off via knock is more difficult since you have to be very precise with the location, but it's not too bad.
I used it with Nova for almost a week and this morning I reverted to the LG UI, just to see if the knock is easier with the OEM launcher. Well... it is MUCH easier, but like others said, not sure that is a good enough reason to ditch Nova.
Nova is great and unless I am using the One, the first thing I always do is replace the launcher with Nova and the stock keyboard with SwiftKey.
What I did like about the LG UI is that you can put more than 4 apps in the dock, just like you can with Nova. That alone made the transition to stock UI much easier.