DOS2 is way harder. BG3 relies a lot less on environmental effects(though they still exist) and I feel that there is a lot less movement abilities thrown at you.
I felt like every time I made a plan in DOS2 the enemies would just cast fly and get out of it. There's a lot less of that in this game.
DOS2 felt like a puzzle game after a certain point where you had to have the right spells to manipulate the battlefield in a specific way but this game is a lot more free form with the encounters.
Been playing on medium and while it was difficult at first I feel right after you start act 2 the game gets a lot easier with all the abilities your party has at their disposal.