The main thing I find (Xbox One X) are inconsistencies between circuits. Some look fantastic, Road America stood out to me after years of playing the Forza version for example.
Screen tearing is pretty rough at night, in Multiplayer and on street circuits. The lack of anti-aliasing is pretty noticeable in places too. According to
Digital Foundry the game runs with a dynamic resolution, even in Prioritise Framerate mode, but never drops to 1080p.
Forza Horizon 4's framerate mode locks the game to 1080p but also helps guarantee 60fps. That's something I'd like to see, as generally everything else about the game I am enjoying