Well to talk about balance, firstly this is early access and this is probably the least balanced the game will ever be because fine tuning that balance is part of the point. Second, yes you still need a tank in pillars of eternity but the point is because you have a large party you can take a tank, a healter AND STILL have room to bring along three other characters that can help but are primarily there because you like them or think they're interesting from a mechanical perspective.
Let me give you another example. The game Greedfall. It's an action RPG, you get 6-7 possible companions and you can only take 2 at a time. Much as I'd like to be able to take more companions at a time because I think they're cool and I like spending time with them, I never felt like I NEEDED to take a specific character to get through the game. It was constructed in such a way that you could flexibly take any character and while there were certainly some situations where a fight was particularly hard and I felt the need to have a specific party makeup, those moments were few and far between, and I could generally take any combination and be able to get through 90% of the game. Because of the way D&D classes work, you can't do that until you have a really good grasp of strategy and tactics and all sorts of stuff like that. And the only way to get that grasp is to play and practice.