Assume at your own peril.
Some games' use of PhysX is not optional. If you're running on an AMD card, then it's just offloaded to the CPU. Ask anyone who's only ever owned AMD cards why they have a mysterious PhysX installation folder on their computer. And ask them what happens if they delete it and then try to run certain games, the game won't run.
Under DX11, most of the work is already being offloaded to one core, add the latency for that core to calculate PhysX, and it effectively stalls the GPU. DX12 could help with this, once devs actually code for it from the ground up.
Then there's the whole bullshit around Nvidia forcing ridiculous tessellation levels (64x) on objects that get zero noticeable visual gain from it. This is because Gameworks titles will handle the ridiculous tessellation better than AMD, so the performance penalty will be much higher on AMD cards.
And I've done some digging, yes, Nvidia has fucked performance on older gens of their own cards. Witcher 3 fucked Kepler hard. A 960 outperformed a 780 ($200 card beating a $700 card).
Really though, I'd do a "lmgtfy" for "why gameworks sucks" but I can't even be bothered. Do your own homework, you don't need to be spoon fed.
For your transgressions you shall be labeled a shill, called an idiot and anytime you mention facts or disagree with a tribe member you will henceforth be known as a troll. The best you can hope for is that the labels won't haunt your offspring. -RedEye9