I have the Steam version and can confirm that copying the folder elsewhere works fine.
I have my Steam library on my HDD, where the game is installed and where Steam updates it and all that, then I copy that Witcher 3 folder to the root of SSD to play it. NO problems at all, and loads way faster.
Dragonbird is right that moving programs around like this rarely works without problems, but in this case it works fine. Steam even knows when I am playing the game even though it isnt in the Steam library folder anymore.