One that comes to mind is cutscenes. If something was initially designed with 16:9 in mind, expanding the FoV or aspect ratio could reveal parts of the image the devs don’t want the player to see. For example, using 120 FOV at 21:9 in Fallout 4 makes the edge of the camera clip through walls sometimes.
The solution is just designing it with ultrawide in mind. Ultrawide owners are a pretty small part of the gaming market overall, so it’s not surprising they don’t do it.