I think it depends on how you approach the game. I'm fairly obsessive when it comes to completing things, so I've been mopping up all the side quests/missions as soon as they become available, meaning I've been leveling up quite well. Consequently, I've yet to run into anything that's given me a problem at around 30 hours in.
However, a mate who's really just going for the the main storyline has been utterly struggling each time he hits a boss fight, and has then had to go off and either do some side quests to level, or just grind a bit, whereas I've had no need to grind because of clearing up the side quests.
I think that some reviewers (those who marked it down) played it the same way as my friend, as they seemed to think that you had to go and grind in order to progress through the game. I'm betting they just stormed through the main storyline in order to hit the review deadline, then moaned that you had to grind simply because they ignored the side quests and were therefore under-leveled.