Tight restrictions force you to be more creative. You don't always have the option to pick the best option with no downsides when faced a problem. Even in Original Sin 2 you probably had on average 3-4 ways to solve any given problem. Your playthroughs will actually become different, since you don't have a Wizard at hand to dispel a magical barrier every time, but have to find another way around. I highly doubt that the game difficulty requires you to have a minmaxed superparty obliterating everything on first turn. Getting through the game with an ill-suited band of misfits failing every other d20 roll might just be the greatest adventure you can have.
Fighters don't have as much utility, but Larian is adding plenty of stuff you can do besides swinging a sword even if you don't have any class features for it.
