I'm going to bring up something important that is easily overlooked:
Spells with damage dice as written in the rules are designed to be used by PCs rather than monsters, because the hit point values are calculated differently.
As such, if using monsters with spells, keep in mind that if you don't adjust the damage dice, the spells may deal more damage than monsters of the same CR are meant to deal in a balanced system.
Less "easily overlooked" and more "something people do not wish to accept as true".
People want to believe that rules designed for use
by PCs can be used perfectly fluidly
against PCs with zero problems whatsoever, and that's simply not true. Mages you fight in combats get to unload their entire suite of daily resources in a single combat because
why wouldn't they, it's a resource they have and they want to succeed. But that makes them
ridiculously more powerful than an equivalent melee combatant type, because they're unloading resources designed to be used across dozens of rounds, not five rounds.
The problem is, the design D&D has used for character power is built on assumptions which run counter to this principle of symmetry. Namely, the idea that some categories of strength are built around
long-term consistency, while others are built around
shepherding resources.
It's perfectly fine to have a design where all rules work identically for all game-entities, including opponents. It's perfectly fine to have a design where some entities are totally or almost totally consistent-baseline-performance and others are totally or almost totally "I decide when and how much power to deploy"--difficult to make actually (=interestingly) balanced, but perfectly fine in principle. It is not fine to try to blend these two principles together with a third: most opponents will be fought only once per resource-cycle. Even BBEGs that you do fight again later are almost never fought twice in the same day, and thus monsters built by the same "I decide when and how much power to deploy" rules will be
grossly overwhelming when used back to back to back in a single combat.
Two assumptions and the natural pattern of play. Something has to give, either we:
- Get rid of the massive ability to dump a day's resources into a single fight (thus making casters no longer "daily" resource-based)
- Get rid of the requirement that all entities (PC and NPC alike) work by 100% identical rules, so that just NPCs are denied the choice of dumping a day's resources in a single fight
- Break the natural pattern of play, either by actually forcing caster NPCs to fight multiple times in the same day, or by (randomly?) reducing the total resources the NPC-built-like-a-PC has, so that they have about as much as they should for a single combat
Unfortunately, I have seen no evidence that this is likely to happen anytime soon.