There have been rumors claiming that the Xbox Series X runs the newest Assassins’ Creed title, Valhalla at 4K native with solid 60 FPS. While the latter is true, it turns out that the resolution is dynamic, rather than native 4K. (Theclickreader) That means that the game will automatically reduce the internal resolution if performance hits a rough patch.
Furthermore, the graphics settings are more or less on par with the PC High. While this is not a bad thing in particular, many players have been exaggerating the issue, claiming that it was due to the superior GPU on the Series X that allowed native 4K.
In general, you’ll see most developers optimizing third-party games for the same resolution on both the consoles, despite there being a delta in the overall graphics grit. You may call them lazy, but most AAA developers are already short on time as the release draws near, and optimizing for a console separately can day up to several days.
There will be shorter loading times and a good frame rate that make the game a lot more comfortable to play, as well as stuff like better draw distance, dynamic 4K, and so on
Julien Laferrière, Producer (Source)
The same was also confirmed by Tom Warren, who believes that the setting is on par with the high preset on PC.