Unless the GeForce now servers were co-located to your local ISP, it's hard to believe. Even then, you're adding quantifiable jitter to the equation on the order of a few milliseconds. It's noticeable.
I’m not really a casual gamer either. I’m still surprised that GeForce now doesn’t have an app for Linux. I switch to my Mac laptop to play stuff like Fortnite.
Fortnite is about as casual as it gets. You won't rank very high in a twitch shooter like CS:GO (or keep consistent ranking) using a cloud gaming service.
Just because one game has guns doesn’t make it less ‘casual’, which btw is a dismissive word to use in the context. Why are ‘twitch reactions’ less ‘casual’ than puzzle solving?
Plus cross platform.