After finally having finished it, I gotta say that the game becomes way too easy towards the second half and by the time you reach Hunter's Edge, all of the enemies kinda stop posing any sort of threat to your party. There are several reasons for that:
1. At some point of the game you just start getting way too many useful tools. While at the start you are pretty limited in terms of what you can do, towards the end almost every character has everything - damage, crowd control, utility, healing, defensive spells, etc. You have lots of CC spells, which can be used to keep several enemies out of the fight almost indefinitely, lots of healing to quickly recover from any bad situation, shields and resistance buffs that make your characters very hard to kill and enough damage buffs to make your 2-handed warrior one shot a major boss. It's also very easy to max out all your elemental resistances, so your characters can safely disregard most forms of damage. Also, apart from AP, which quickly regenerates every turn, your spell casting is not in any way limited and you can spam heals and CC however long you like. Personally, I like when the game gives you a lot of tools to use for your characters, but I feel that in this case the enemies should be given a lot stuff to deal with you.
2. The AI is very stupid. I mean VERY stupid. It just does whatever it feels like without taking anything into account. I saw the enemies waste turns running around my melee fighters provoking lots of attacks of opportunity, refuse to cross a tiny patch of burning ground that blocked the way, suddenly switch targets instead of finishing off my mage with 100 hp left, waste powerful spells on shitty summons, waste powerful spells on something that is immune to them, etc. Only once in the entire game did I see an enemy caster using Purifying Flame to remove my elemental shield before attacking. 99% of the time it just does things at random.
3. The rate at which you gain HP vastly outscales the rate at which your enemies gain damage. During my early levels my characters ran around with about 200-300 HP, while some enemies can easily hit them for 100-150. In the late game, my characters had between 1200 and 2000 HP, with the enemies being unable to hit them for more than 200-300. The problem is that enemies get no new skills as you progress. Most enemies in the second half of the game waste turns to cast useless crap like Curse or Blind or Headvice, which at that point are literally useless. Why can Braccus Rex, the final boss of the first "act", cast Meteor Shower, which at that point in the game can instantly kill your entire party unless you spread out beforehand, but no other enemy that comes after him can?
The later game enemies don't get any unique abilities or strong weapon attacks that would allow them to quickly eat through your HP and one Meteor Shower with a damage buff can remove half of the enemies in a group before the fight even starts. The only enemies that did something were Death Knights - their damage was decent and you had to deal with the immortality thing, but due to point #1, my party was never in danger anyway. Same with bosses - many of them had unique abilities, but usually they were crap, like summoning a few minions that die on the same turn they were summoned. And apart from that they do the same things regular enemies do - waste several turns giving your party curses, blinds, weakness, AIDS, etc., and then try killing them with 100 damage elemental spells.
Despite this, somehow I never felt that the game was too boring to stop playing, but it's really a shame that the difficulty dropped so much, cause it was reasonably challenging during Cyseal and its surroundings. Still, it's a great game, a solid 8.5/10 and probably the best RPGs released in years. I really really hope that if there is a Divinity OS 2 or an expansion they will properly balance the difficulty to last for the entire game.