I can kind of see why Crius is concerned in terms of the dungeon being too decisive a factor in character progression (mainly in terms of it possibly making the main quest too easy if you gain too many levels in the dungeon) but isn't this a problem that you'll always have with RPGs that lack level scaling? The main quest will always be geared towards player characters of a specific range of levels. In most games (that I've played) the main quest has tended to be something that proves relatively difficult (if not impossible) without taking part in a certain amount of levelling unrelated to the main quest, either through sidequests or general grinding. But on the other hand, too much levelling outside of the main quest will make it too easy. That's just how it is with CRPGs.
In this specific case, yeah, the dungeon offers a lot of potential for gaining experience and loot, and it has the potential to make the main quest too easy... especially if you feel obliged to complete the entire dungeon before finishing the main quest. (Which maybe you are, I'm not actually sure that a post-main quest endgame state is confirmed.) But (hopefully) you're not. This is the beauty of CRPGs, you're not obliged to do anything. If you feel like too much time spent in Od Nua is making your party too tough to fully enjoy the main quest, you can just ignore it for a while. Likewise if the main quest is kicking your ass too hard (and I hope to god that is a strong possibility in PoE) you can head on over to the mega-dungeon, or any other source of exp and loot, and improve your party a bit. The only way I can see this being a problem is if you personally are the kind of completionist who has to do as much as is humanly possibly in each playthrough. If you want to hit up every side-quest, dungeon, map location and optional source of exp in the game then obviously the main quest is going to be easier for you than it would be otherwise. But you don't! This is a CRPG, you can see all there is to see and do all there is to do over a multitude of different playthroughs, with different characters, choices and builds.
I personally will probably have one playthrough (probably as a more generic neutral good character) where I focus entirely on the main quest, and another (maybe as a more mercenary, self-serving chaotic neutral character (excuse the DnD terminology, it's inescapable)) where I maybe ignore the main quest entirely and plan my entire game around conquering Od Nua. That kind of variety, to me, is what I find attractive about the genre. And it also allows me to be a completionist and maintain a sense of difficulty. If you want that difficulty while also trying to accomplish everything in a single game... you might be more likely to be disappointed.