In my evaluation these values seem pretty reasonable, even with some min-maxing, and not extremely abusable.
[EDIT] [Draco-demi-vanilla] liches (and other potential undead casters) rule the earth
They are immune to exhaustion, and potentially cast 9th level spells continuously and at will.
If BBEGs can do this, Legendary Resistance is more powerful
This means a lone caster that can cast 8th / 9th level spells can invoke an additional 9th level spell once every 5 days without risk of dying (and if they have greater restoration, with little consequence).
It makes Potions of Vitality (only Very Rare) amazing
Removing all levels of exhaustion lets you use Revivify to top yourself back off.
It allows a caster to build very tanky with less implications
For our hypothetical problem situation below, we'll be casting 9th level spells, e.g. Wish, Meteor Storm as much as possible.
It allows converting 7th-level slots into extra 9th-level slots
Per https://rpg.stackexchange.com/a/53555/38563, we assume that Resurrection removes all levels of exhaustion (though imposes a -4 to pretty much everything afterwards).
At the extreme, Resilient and 20 Con and +6 proficiency means +11 to saving throws, and 100 extra hp.
Lets see how many 9th level spells we can pull off before burning out:
Number of casts in pseudocode:
roll = d20 + mod
if roll <= 14: return 0 # A
if roll < 19: return 1 # B
# Have 5 levels of exhaustion, not dead (but disadvantage)
roll2 = min(d20, d20) + mod
if roll2 <= 14: return 1 # C
return 2 # D
In terms of probability, this is (for the ranges we care about)
p(0) = A
p(1) = B + C
p(2) = D
A = (14 - mod) / 20
B = 4 / 20
C = (1 - A - B) * (1 - (1-A)^2)
D = 1 - A - B - C
Approximate casts before death:
Mod | 0 | 1 | 2
----+-----+-----+----
+11 | 15% | 38% | 47%
+7 | 35% | 46% | 19%
+3 | 55% | 40% | 5%
-1 | 75% | 25% | 0% (0.3%)
-5 | 95% | 5% | 0%
Expected casts in the 5 lives before instafails (-6 or more)
9th level spells for free | Chance
--------------------------+--------
0 | 2%
1 | 10%
2 | 24%
3 | 30%
4 | 22%
5 | 9%
6 | 2%
7 | 0.2%
8 | minute
9 | vanishing
10 | implausible
The average is 2.963625 (exactly 23709/8000, as it happens).
To run the above you would need a level 20 Cleric spending all day (4 hours) and energy (level 7/7/8/9 spell slots) reviving you, or 4 level 13s, or other equivalents, unless they wanted to roll the die themselves.
If you consider True Resurrection to remove this penalty (could not find any discussion on this) and you have no Wish penalties accruing then you can rinse and repeat.