The DMG has some really useful guidelines on creating monsters and assessing their challenge. But they seem to assume that all monster attacks will key off their attack stats or spells, and completely ignore the possibility of damage that does not require dice rolls (or that happens on a failed dice roll).
This can have considerable effect on the Challenge (CR) of a monster, and has tripped me up more than once. Are there any additional guidelines on how to calculate monster CR when it relies heavily on automatic damage or on effects with a save for half damage?
A good example is a comparison between a monster that has a normal attack that hits for 3d6 damage, versus a monster that is capable of casting magic missile. Both monster would have an expected damage output of 10.5 average per round, tagging them at Offensive CR 1. But the caster is vastly more dangerous because it will definitely deal that damage each round, whereas the other monster likely only has about a 50% chance of hurting anything.