I feel like the terms serve little value. Partially because casual is less a category a more an insult.
As far as content goes I can't really see Extremes working as anything other than above "middle". The first ones a new player gets access to can be very demoralizing given the leap in difficulty. Ramuh EX alone is just a wall if people haven't properly understood mechanics that most players at 90 would consider baseline.
-Don't stand next to tanks
-Pay attention to personal aoes
-Pay attention to the debuffs of other players
-After certain moves don't stand in specific map sections with no aoe telegraph
-Prioritize adds
Then it also has a tank swap and a tether that you don't get rid of with distance changes. That's a lot compared to every dungeon and trial before it.