I've been playing Fallout (and Sim Settlements) on the X. It works and looks great, which is not surprising since it has something like a GTX 1070 equivalent. Fallout support dynamic resolution on the X, so you can see it kick into 4K in the interiors, while outside my guess is that it going at 1080p/30fps, maybe faster when you're not downtown.
As far SS is concerned, I've only been messing with RotC for the last few days and it's fast. No slowdowns, the foundations get laid down in a minute...