@GoanGeek I have the xbox ultimate pass and cloud service on my S21U. It all works fine, and games play fairly well for using a phone platform ... of course the 16GB RAM and fairly powerful processing help with the games.
However to play a game on the phone screen it is somewhat of a novelty... ok for a quick game, but ultimately it is never easy to play an FPS or adventure game on a 6.8" screen. The screen is too small... or maybe it is my old age eyesight 😂
A little tip... if you have a smart TV and can screen share from phone to TV, you can play cloud games in large screen... and actually with a pretty decent performance. It is not going to keep up with an XBOX or gaming PC, but it is actually quite playable. Graphically the phone is capable in WQHD+ on the phone screen, and this give a fairly good graphical output to a 55" TV !
I use a Razer Kishi gamepad... which with a bit of adaptation works perfectly with the S21U in a slim case.