Mummy Lords and CR

[Trask]

Yup.

The CR system is predicated on the players winning 100% of the time, even against Deadly difficulty. One of more players might go down. Before your first level 5 hoard, you might even lose a character to permadeath that way. But there is 0% chance of a TPK if you use the DMG guidelines, and almost no chance of a character actually dying. That's very intentional.

It's also designed for 3-6 enemies per fight, for 4-6 fights per day. Also intentional.

There's no reason to judging the CR system lacking due to a single very high CR creature that successfully TPK a party, as it should. There is no point in judging the CR system based on extreme outliers in value, either lots of very low CR or a single very high CR (relative to the party), at all. Of course it's going to break down at the edges, especially in a system where lots of weak creatures are intentionally designed to be a threat to single powerful creature.

There is a point to saying "the CR system doesn't do what I want it to do". But given most often posters saying that actually want a string of equal CR solo creatures vs four adventurers to eventually result in a TPK for the party, once you go to the natural consequences of a campaign full of what they want for a single battle, it's unreasonable to expect that that designers would have used that as a baseline.

Truly? It's designed for 100% victory? That's pretty crazy to me because I have lots of deaths in my games and I don't go ridiculously outside the bounds of suggested CR.

I don't like the idea of a system that is designed so the players will never face anything seriously dangerous, but I suppose that reveals how out of step I was with the CR system in the first place.