The game isn't too hard from what I played of the 2.1 endgame stuff (T5, Extreme Garuda/Titan/Ifrit) but it's still challenging. There are very few "you made one mistake and you're dead" situations like in a lot of other MMOs I've played, though it does have a few instant death/wipe mechanics in some fights.
All the max-level dungeons are pretty easy once you see them at least once. Most boss abilities are telegraphed based upon what trash mobs in the same dungeons do, so if you just pay attention while clearing trash, you can generally understand how to deal with the 4-person dungeon bosses.
A lot of times, making mistakes doesn't wipe the party or kill you instantly, so you can recover. Even on Extreme modes, or at least the ones that I played. That doesn't mean the game is lacking challenge... it still pushes people in every single role. It just isn't built around pressuring healers/tanks an unreasonable amount.