Check out this explanation by Linus on Techquickie as to why there are SLI and CF scaling issues. I am a fan personally of single card setups because not every game or application will use both video cards. I only recommend SLI/CF once you already own the most powerful single GPU and have no choice but to add another. Click the video to play it or the source link below.
Jay explains the truth behind the false rumors of Pascal only being able to do 2 way SLI. Basically you can do 3-4 way but will need a key from Nvidia. Nvidia technically only supports 2 way now but they are not locking you out of more cards. Click the video and give Jay a follow!