Both. Node crash on launching with it.
Edit: And for some insane reason it's now not exiting properly again, game still runs using various methods to close it, including CMD + Q, Browser -> Close, Windows X etc.. I suspect it's Steam's overlay trying to inject itself into one of the nw process and this causes issues. Hopefully they fix --single-process arg soon, I'll test again then.
Edit2: Can confirm it's the Steam overlay injection. I disable overlay and the game exits just fine!