Should undead be immune to Exhaustion

[KorvinStarmast]

One thing that I learned about D&D undead talking to some of the earliest players -- when first conceived, undead were not immune to each other's drains. If an undead was hit by another undead, it would lose HD (and, because why not, change what type of undead they were). This lead to a playtest scenario where two groups of vampires started dueling (Sharks vs. Jets style) high above the ground. If a Vampire was hit, it would turn into a Spectre. If a Spectre was hit, it would turn into a Mummy -- and plummet to its demise. Honestly, the game needs more absurdity like that.

That it does.

[Mellack]

While I believe the specific volume is not from the rules, it is still interesting to note that a gallon means a vampire either kills one person each night, or has a stable of around 450 donors who he feeds regularly but safely on. The latter case requires roughly one pint from each of eight donors each night, with 56 days between any given donor providing a donation.

If we instead allow vampires to drink water and say they only need the blood as food, about 1 pint of blood would equal one pound. That makes their requirements much more manageable.

[greenstone]

Oh, another interesting fact, and one that I was unaware of for quite some time, is that undead aren't immune to disease. I'm not sure any monster is.

As if undead aren't gross enough already.

GM: The zombie advances, bits of rotting flesh falling from its corpse as it lurches forward. Suddenly, it sneezes on you! Bits of, well, you don't want to think about what they are bits of, are now all over your face.

*evil grin* Next session, I'm going to have the PCs encounter ZOMBIES WITH COVID-19!!!!

[Zhorn]

If we instead allow vampires to drink water and say they only need the blood as food, about 1 pint of blood would equal one pound. That makes their requirements much more manageable.

In regards to volume I agree and I think that is a very reasonable quantity, but I still think trying to apply rules for daily food/water needs across the board on all creatures is a mistake.
Less of a RAW matter, and rather just because it can get in the way thematically for some monster ecologies, and the general fiction in other areas.

Vampires as specific to the conversation has a common theme of going into a deep sleep/hibernation while food is scarce, and having a hard limit of feeding minimums gets in the way of that.

Other creatures have lore relating to eating habits that also don't mesh with the rules. Mind Flayers for example don't need to eat every day, and can get by with a minimum of one brain per month without suffering debilitation.

A mind flayer must have a minimum of one fresh brain per month. Any less than that and it suffers physical debilitation, becoming so weak that it could die. Its ideal diet is one brain per week. A mind flayer that consumes one brain a week does not feel deprived. It can eat more than that for enjoyment and for the psychic boost, and it will if brains are plentiful.

[Greywander]

"Guys, we need to ration our brains," seems like a great out-of-context quote.

If hibernation is a thing, then it could be written up as a trait that allows the creature to pass large amounts of time without needing to eat or drink as long as they remain asleep. Then again, this is starting to get a bit too detailed, and would likely lead to needing to have a rule for every little thing.