There are a few spells and other features that can automatically deal damage, but it's very rare. The most common example I can think of is magic missile, which strikes unerringly and deals its damage with no attack roll or saving throw involved. Some area effect spells like cloud of daggers also deal their damage with no attack roll or saving throw.
In these particular examples, I see mitigating factors: for magic missile, the property of the shield spell to negate magic missile provides a way to counter it and the low damage dice seem to trade damage magnitude for reliability; and for cloud of daggers, a creature usually has to choose to move into the 5 foot area of the spell to take the damage, so it's mostly good for choke points.
However, in these particular examples, I also see counterpoints to those mitigating factors: very few targets are going to have the shield spell, and magic missile deals force damage, the least likely type for a creature to resist or negate; and cloud of daggers can be cast right on top of a creature to deal its initial damage without any practical way to be avoided.
Now, these two spells are just examples of the kinds of spells and features I'm talking about. There is advice in the Dungeon Master's Guide for designing new spells (p. 283 to 284), but it's very brief and doesn't seem to address this issue. My question is: How are these sorts of features (those that automatically deal damage) to be balanced relative to other features of a comparable level or categorization, based on official guidance or an analysis of the apparent techniques used in official material?
An alternative way to phrase the question with (as far as I can tell) an identical meaning is: "When is it balanced to not make an attack roll or require a saving throw to deal damage with a feature?" (In case I'm exhibiting the XY problem, my intent is to make a monster feature that automatically deals damage without violating the balance of the game, without simply copying magic missile, and without limiting my understanding of the balance issues to a damage-per-round calculation in consultation with the monster statistics table in the DMG. I would like to know how to balance such a feature without assuming it's for use by a monster.)