I don't have any noticable input lag but I'm on 70/30mbit fiber with 3-4ms ping to stadia.com. Sometimes the stream quality drops for a sec, but no noticeable frames or audio dropped.
I agree with the darkness being to bright, couldn't fix it with the gamme correction either. I have a feeling the 4k does a little better job in that regard. But overal it's not bothering me to much.
[0] https://chrome.google.com/webstore/detail/stadia%20-extensio...
Depending on how user input is processed in a game, this problem can be mitigated.
The lag is not too bad, but still noticeable. I only play on the PC now. I have no idea how to fix WiFi lag.
https://community.stadia.com/t5/Stadia-on-Chrome/Fixing-stut...
They do recommend 5Ghz for Wifi and being as close to the AP as possible. I can image if your AP is behind a wall or in a closet or different floor you get noticable delays as signals come in via reflections rather than directly.
I'm also playing Cyberpunk 2077 on Stadia and I've been quite positively surprised by it. However, I'm not really sold on Stadia generally yet. There's definitely some input lag and I can feel my mouse movements being "spongy". I'm also having a hard time driving, but I don't know if that's the input lag or just the game. Also, even if the game is running at the best possible graphical options (which Cyberpunk 2077 isn't), you're still just getting a compressed video stream, so dark gameplay doesn't look good (https://0x0.st/iFRT.png).
No regrets though, since I paid 50€ for it and I'm getting a free Chromecast Ultra and a free Stadia controller.