Intellect Devourers, Banshees, & even Shadows can be very deadly. Undead with nasty abilities generally seem under-CR'd.
I critted a character with a wraith not long ago, and it did so much horrific damage and capped the cleric at 5hp.
Unfortunately, all of them do both, depending on the party and the environment.What monsters do you think overshoot their CR in 5e? That seem too powerful for their respective CR or should be moved to another CR entirely?
Similarly, what monsters do you think are too weak for their CR in 5e? That seem to belong to a lower CR entirely?
Or, although an issue in earlier editions of the game, the mechanics for monster design and CR assignation is tight in 5e and the monsters as rated prove the appropriate amount of challenge?
By the monster design guidelines, Orcs should be CR 1 (defensive 1/8 + offensive 2 = 1).Orc is a weak CR 1/2, but just squeezes in due to the +5 to hit bonus.
I had a similar experience, wraith dropped a level 5 elf wizard from max hp to max hp = 0 in one blow!

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.