Helldritch
Hero
The mistake of 3ed was not to have NPC behave like PCs. It was to not give a enough example from which to build from.This is why, in the end, I understood why 4e, PF2e and similar moved away from treating NPCs/monsters similar to PCs, even though, coming from a Hero and RQ background I don't like it; things that work for PCs in terms of complexity are a nightmare when you're trying to manage them as a GM, as I learned while running D&D3.5 in the low teens.
(There are other issues where people have carried over expectations from the 3e design era--which after all, PF1e was pretty much an extension of--that aren't too benign either, but, well, people want what they want and not everyone who has these are players).
Take the dragons. Only a few are fully build up. If the dragon you need isn't the one fully described, then tough luck, you'll have to build it from scratch.
5ed learned the lesson (as 4ed did) and gave us one example of each dragons in the book, all age categories included. The same goes with NPC humanoids. With the veteran, I can just change the race to "orc" and I have my veteran that I need. For the mage all the spells are there and I can change them. They expanded on that in MtoF and VGtM and gave us various levels for almost every classes in the PHB. Want a gladiator in better armor? Easy, put plate and shield and here you go.
3ed edition's way of doing monsters was tedious, yet, not so once you had a small data base on excel (which I did). But it was not at the reach of everybody and not everyone had a portable computer to access their data base. 5ed is way quicker and people learned from 3ed to quickly reskin monsters into what they needed on the fly.