I kind of like the simplicity of NPC classes and non-classes, but there are basically zero situations where it's better for a human to progress as a Warrior instead of Fighter, or a lizardfolk to progress as Humanoid instead of Monk--and those aren't even particularly good classes.

There's a table for monster CR that gives some approximate values for different types, and what it says makes a lot of sense. Also, world generation guidelines tend to give characters with NPC classes more levels than those with PC classes. So it seems like, for example, NPC classes should give faster progression than PC ones, with the downside of having weaker levels. But I haven't actually found any rules that suggest anything beyond DMG's "don't give players NPC classes except maybe Expert" and Savage Species' "monstrous humanoid isn't that bad, and hey you don't take a multiclass XP penalty!"

Has anyone had a game with either NPC classes or raw RHD progression for players? How was it balanced, and how did it go?