I played Planescape Torment on Steam, and i have got to say, while the game is fantastic, I never really felt like i was faced with a challenge. Meamwhile both BG and BG2 can be brutal if you don't know what you are doing. There are a lot of enemies with resistance that you need to dispel etc.
Well, PS:T is story-focused, while BG 1-2-ToB is balancedly story AND battle focused (Icewind Dale being the other end of the spectrum being clearly battle-focused).
In PS:T you have almost no battles, and they are extremly clvl-adjusted.
The BG-series on the other hand solves problems with battle instead of witts. Your witts in BG come what quests to pick, which fights to choose.