Frequent changes make evaluation difficult.
Going through Guild Wars 2 a second and third time, I am finding it easier. I do not know how much of that is familiarity with the particular content, better knowledge of game mechanics, more experience with a class, playing a different class, having better equipment, or changes to the game itself. Those game changes could also be subdivided into changes to the content, game mechanics, or classes. Those could be further subdivided into bug fixes, intentional adjustments/rebalancing, and unintended effects of other changes. In group content, multiply all these by your number of players.
I know that some early complaints, mine included, came down to whining that the game got hard at the end. Orr and the dungeons differ, with a difficulty curve like Psychonauts (not, blessedly, Psychonauts at release). The same tactics that thrive in Plains of Ashford events will get you killed walking to a cypress sapling in the Straits of Devastation. I am confident that Orr is easier due to learning, but I do not know if the other factors are more important.
Underwater content, for example, is still ugly on my elementalist. I have gotten better at it, but the class is just lousy in the water. My ranger has few problems, with higher DPS when I hit 1 and AFK than my elementalist sees using all 20 skills. My guardian is at least as effective underwater as on land; spear’s 2 is just ridiculous. I am confident that the difficulty of underwater content varies with class, but there are other factors also in play.
These examples are from Guild Wars 2, but I did not use the [GW2] tag because this happens with all games that rebalance, particularly when they update frequently. Sometimes I feel really awesome on patch day, only to discover that some mobs were nerfed. Sometimes I return to an old game and think I have forgotten how to play, only to discover that “how to play” changed while I was not looking.