I don't care about screwing up balance but I very much do care about screwing up in-setting consistency. Thus, I do my best to make sure my ad-hoc NPCs (and many of them are) still fit within what the char-gen rules allow.
Despite my firm advocacy for NPC design freedom, I do actually relate to this. I don't love a lot of the 5e NPC design because it's so wildly out of bounds of what a PC can do. I would never have a halberdier level 8 town guard with 5 halberd attacks per round, for example.
I have some loose constraints I use for humanoid-type PCs, the kind that would generally be expected to have some kind of level and "class".
1) 1 Hit Die per level, generally a d8, but warrior-types might have more and caster types less. They also always have at least 2 proficient saves and 4 proficient skills (if I bother to assign them for combat mooks).
2) Level 1-4 gets 1 base attack, 5-10 gets 2 base attacks, 11-16 gets 3, and 17+ gets 4. This is the baseline for their normal "swing a weapon" action, but a focused warrior type could have maybe 1-2 more, depending on their abilities.
3) The max spell level will generally not exceed the normal full caster progression for a combat-capable NPC. No 5th level spells unless they're level 9. NPCs who violate this rule are either non-combat NPCs (a level 3 healer with greater and lesser restoration as their only spells) or heavily specialized, generally with much weaker hit points and defenses.
4) Besides that, NPCs get the equivalent of about 1 "feat" per level, which justify extra defenses and special attacks. This is a rough eyeball of the potency of special abilities, nothing formalized.
5) Important NPCs will also magical items and/or boons which function as magic items, roughly commiserate with what I would expect a PC of that level to have.