Do you have data to back that up? I've found that when comparing SSIM and PSNR, NVENC's H.264 encoder is able to quite handily beat x264 for quality while targeting the same bitrate and encode speed. This was using a Gen 2 Maxwell GPU and comparing to a mid range Xeon.
It's possible software is the problem here. I have certainly never seen a way to get acceptable quality out of NVENC in common consumer game streaming software like OBS, and I've tried at length including adjusting profiles and tuning settings. It doesn't help that NVENC's documentation is atrocious.
NVENC also mishandles color spaces considerably (full vs limited range), which is a big handicap to begin with.
SSIM and PSNR aren't really the question here either, the issue is totally perceptual. I do not doubt that on a speed basis NVENC is competitive or outperforms, the question is simply whether you can deliver something watchable that hits under the user's upstream cap. If you look at average twitch streams of even mundane video games, many of them are borderline unwatchable - this is a pervasive problem :-) Kicking butt on image-wide SSIM and PSNR won't be much use if all the text in a story-driven game is unreadable due to artifacts.
