It is interesting: I'd want to see the final Exhaustion rules.
Yeah I think a big question is how easy is it to dump Exhaustion and what exactly does it do.
It seems like exhaustion is the -1 to all rolls per level of exhaustion that it was in UA at some point. And I know some people here have certainly thought of having such a house rule, if exhaustion was stacking -1's.
The big problem with the -1 to all rolls per level of exhaustion, which most people seem unable to process, is that it ruins martials and skill users, and has basically no effect on casters.
Like, martials - they have to roll to hit, and so this means they start missing a lot, their DPR and combat effectiveness plummet.
Skill users have to roll skills to act on the world, and so this means they start failing skill checks more and more, and become increasingly unable to impact the world or succeed at anything.
Casters though, retain close to 100% combat effectiveness. Their DPR and ability to control enemies doesn't go down - because they don't roll - enemies do. Basically only their own saves against enemy stuff are penalized - and I guess the few attack roll spells, but the only "caster" reliant on those is Warlock, who isn't a full caster.
Likewise, casters acting on the world through spells never have to make checks or rolls, the spells just succeed, so they can be on completely exhausted, and still able to function extremely well.
As such, this means exhaustion is a huge problem for martials/skill users, and not really a big problem for full casters. That's pretty crap.
Now if exhaustion caused you to lose spell slots, or to have to make a CON check to even cast a spell at all, perhaps that would start to make more sense.