Two Sets of Rules, the Good and Bad?

[KorvinStarmast]

Having monsters and NPCs use different rules from PCs is one of the best parts of 5e, and a major factor in making DMing it tolerable.

Yep.

Bravo! A cogent statement of why Legendary Resistance is a bad game mechanic.

Disagree with "bad" - it meets the 'good enough' standard. I understand why some folks would prefer something different.

I remember how disappointed I was as a player the first time I saw a Mind Flayer die to an upcasted Fireball. I couldn't believe that that idiotic strategy actually works in 5E. Advantage on saves is not nothing, but it's not much compared to 90% magic resistance (as in AD&D). For my games I wanted to bring back the feel of a monster who just shrugs off magic, like water off a duck's back.

I never liked that, the magic resistance (it started in Eldritch Wizardry, OD&D, among some of the demons) but that's a matter of taste. It has now become baked in. *shrugs* It's workable.

IMO immunity isn't unfun as long as it makes sense

Yep, but honestly, not all of them do make sense ...

BTW it's also quite funny that 5E's CR system charges for Legendary Resistance (it's an effective HP increase) but not for condition immunities or teleportation powers. Two ways to skin a cat but only one of them affects CR.

You seem to desire a higher level of precision in CR calculation (you are not alone, there are plenty of people who scratch their heads on that) but I like to repeat this frequently: CR is a ballpark estimate.