I have more than 10 add-on airplanes, none of them has ever caused performance issues or CTD. I bought the Tokyo scenery add-on last week, and so far had some minor performance drops and only one CTD on the first time it loaded, but in general the performance is quite acceptable. I am running the sim on Series S, so I imagine the Series X is even better when handling these contents. Most of CTD I got (only a handful of them, to be fair) were on scenery affected by Asobo’s World Updates and using default aircraft.