Speaking for myself, in a single player game it's not about me building a class X and a class Y, fighting them against each other and expecting each to win 50% of the time, it's about the choices being valid. This goes beyond just paper balance of the classes, but the way encounters and the game is designed. The following examples and discussions are speaking of a generic case to help explain what I'm trying to get it, and are not a specific critic of PoE. Take something like backstabbing, if I want to build a character for that moment of awesome backstab I want it to pay off if the game is letting me build that character. If the class design makes backstab underwhelming, the underlying mechanics make it almost possible to use, or 95% of the game's enemies are immune to backstab then I feel like I've been the victim of a bait and switch (talking about what the game was offering, not that I was monetarily cheated by the game designers). It's a very subjective thing to balance consider you can take something like backstab which tends towards moment of awesome reward and make it feel as valid as a sword and board's high defense and slow and steady damage. It's gotta be difficult to make both choices feel valid when they pay off in very different ways. You even get the difficulty the getting disparate paths paying off within a single class such as a control wizard versus a nuker wizard.
Now issues of validity can be related to, and exacerbated by, issues of difficulty but aren't always the same. If it's more difficult to successfully make a backstabbing rogue pay off that's fine, it just needs to be counterbalanced by a more rewarding payoff (be it mechanical, thematic, role-playing, or so on) versus a choice that has a lower but more consistent payoff.