My understanding is that the end game spawns are a) balanced to be of a challenge rating appropriate for a level 20(+?) group when in reality the player is likely to be in the level 14-17 range and b) require a very specific buff that if you haven't googled ahead you may not know you'll need.
Also, if you haven't googled ahead you'll probably not realise exactly how vital it is not just to complete the companion quests but to complete them with the correct choices/results (or to go full merc), not to mention if you're roleplaying those 'correct' choices may not fit your character at all.
In my case I knew about the over-tuning of the spawns and I knew about the specific buff, but despite doing 95% of the side quests I'd encountered was only level 15 when I got there. I wasn't aware of the companion element so my earlier choices (which in 2 cases were affected by bugs or design oversights - Harrim and Amiri) meant my level 15 party only had 2 companions available, one of which I hadn't been using so was underequipped, and neither of which was the character with that specific buff. Needless to say, after 60-70 hours of playing that particular playthrough hit a brick wall and was pretty much unsalvageable (at least, not without some heavy save game editing).
The main difference compared with the rest of the game is you're no longer in a position to go back and change things to take a different approach, and you can't just go level up and try again later the way you can with earlier over-tuned mobs (and even if I'd had a save pre-portal, the decisions that would strip me of my party post-portal went back much much further).
I'd been enjoying the game up to that point despite earlier bug and balance wobbles, but I'm finding it hard to motivate myself to start over knowing how it is/was designed (even now I know the specific path required to avoid most of that).
(edit: and slapping a 50% spellcasting fail environmental effect on top of all that was the last straw for me, that's where I gave up trying to persevere with my level 15 group of 2.5 characters)