A flat ceiling would facilitate an easier installation, but a cathedral ceiling would still be quite simple as the walls are still flat. You’ll most likely just need to use taller ladders.
A curved ceiling is where things start to get complicated, as you’d need to cut into the back of the panels to allow them to bend.
For a cathedral ceiling, obviously, the installation would be slightly different, but since the panels are so light, it doesn’t really make the installation that much harder. Additionally, a cathedral ceiling increases the amount of space that needs to be conditioned; but it also increases the ceiling surface area, so you can easily increase heating and cooling capacity by utilizing more panels.