There are a lot of conflicting opinions, but I'd say the game can be difficult, but mostly at endgame. Some of the leveling dungeons are a bit more of a challenge than the level 50 ones due to overgearing (levelling dungeons have level sync with a maximum of +3 -- so a level 20 dungeon can be run at no higher than 23).
Most everything is tightly scripted, though some things are a bit looser. Those more loosely scripted items are what tend to kill people the most, though there is a mechanic in turn 2 that has no system side variables at all that most players just avoid entirely because a mistake on the part of any player will kill everyone.
Most of the people here saying that the game is too easy are of the opinion that something can't be considered hard if being good at it means you'll almost always win. It doesn't seem to matter that most people are not good at it, just that fact that there's a way to win 100% of the time means that it's easy.
As others have said, learning a fight to the point where you almost never make a mistake takes a fair amount of time, and of course there are 7 other people with you. Because most mechanics require the attention of several or even all players, 1 person messing up can end a run. Sometimes you'll find the same person screwing up every time and the fix is obvious. Other times you'll see that player 1 dies like once in 5 attempts, while player 2 dies once in 7, and player 3 dies 3 out of 8 attempts. All of these players are getting it right most of the time, but unless their successes are in sync with each other you're going nowhere.
Of course a player getting hit by an AoE or something isn't all that can go wrong. If you fail to meet DPS checks, for example, figuring out why can be a headache given the lack of in game parser and the fact that nobody's going to raise their hand and tell you that they were playing poorly.
TLDR; you can get very far on mostly easy content. Endgame is much harder due to stricter mechanics and the fact that there are 7 other people dancing with you.