Nothing about this game is fun.
The more difficult it is, the less fun it becomes, because everything about it is frustrating. I love difficult D&D-based games - Knights of the Chalice 2 is a favorite of mine, ToEE is a classic I like, and Solasta is cool too. But NWN? Utter shit, because it fucks up the fundamentals.
D&D is a party-based game. If you play a fighter you don't have much to do other than attack, and attacking is slooooooow (once every 6 seconds, MUCH slower than ANY turn based game). If you play a mage you will die in melee and get your spells interrupted by AoO attacks all the time. The solution to this problem in D&D is that you have a fighter to capture the enemies' attention and a mage standing behind him to cast damage/debuff/environment effects for tactical synergy between the two characters.
NWN doesn't let you do this, because it is a single character game. In a system designed to be played with a party.
Oh sure, there are hirelings, but they are AI-controlled and even if you try to give them commands they will mostly ignore them and run off to commit suicide. I played a few chapters of Swordflight, and some of the fights in those modules are genuinely well designed. In any other engine they would be incredibly fun encounters. In NWN, they're an exercise in frustration because of the shitty companion AI and lack of party control.
Also, everything in NWN moves slow as molasses due to the one action per 6 seconds RTwP timeflow. Which wouldn't be that bad if you could control multiple characters... but you only control a single character, so you can only make one decision every six seconds. Incredibly slow and boring.
NWN is shit, 0/10 game, I tried deriving some fun from it many times, but every single time it ended up being a frustrating and un-fun experience. The most enjoyable modules I played were those that barely had any combat and just roleplaying, sorta like Disco Elysium, because at least the non-combat gameplay is bearable as long as you use CheatEngine's speedhack to improve the torturously slow walking speed.