Larian didn't implement exhaustion levels in the berserker barbarian sub class(after rage ends you get an exhaustion level - movement is halved, attacks have disadvantage and so on) - so without its main flaw berserker is pretty OP, as it's the only sub class which receives extra attack before level 5. Funny thing is that in the rules exhaustion disappears after long rest
2 points of clarification+opinion here
1.) Long Resting only reduces a creature's exhaustion by 1 level.
2.) Because of ^, Berserker Barbarian is widely regarded as a terrible subclass specifically because of how punishing getting levels of exhaustion is. Before level ~9 (when the party gains access to Greater Restoration), a barbarian can effectively only Frenzy 1x per day, barring the DM allowing entire days of downtime somewhat regularly. I very much agree with Larian's decision to change this, if not necessarily their exact solution.
As for my general opinion on exhaustionI think Larian should add Exhaustion to BG3, but only as specific story- or monster- or item-given effects. E.g.,
- After you rescue Lae'zel and/or explore the crypt (importantly, BEFORE you arrive at the grove), the game gives you 1-3 levels of exhaustion. This encourages players to rest before they enter the grove and talk to Nettie, which will ensure that they get the *incredibly important* cutscene that reveals you're not actually transforming into mindflayers according to normal ceremorphosis.
- Items can have abilities that, when activated, give a level of exhaustion. Certain monsters can drain your vitality by giving you levels of exhaustion, possibly multiple in a single fight.
Certainly, since you can currently freely Long Rest in BG3, there is ~no point to exhaustion unless multiple levels of it can occur in a single fight.