Forget everything i said, using the xim for fortnite is like weird. Idk its not the game cause when i use the controller its fine.
comparing your experience between a controller and a mouse do not give any valid scientific data. When it comes to electronics, there is almost never a logical answer. I'm not saying your xim isn't having issues, but don't assume it is either without concrete scientific evidence.
Also, if your issues are only happening on Fortnite and no other game, then how is Xim the issue? It couldn't be because the Xim does not change how it communicates with the console when you change games. It is possible the fortnite developers changed something with the movement/aim mechanics and the ST may need to be re-calibrated.
I would first recommend you double check every setting on the Xim to make sure you didn't accidentally change something. I can't say how many times I accidentally increase/decrease a setting when i scroll up or down the screen. It is rather annoying and I wish Obsiv would figure a way to prevent this from happening.