If the adventure is short, what stops the players from adjusting the levels of the characters to account for such discrepancies, or at least acknowledging them if they don't want to adjust? This is exactly the same way that some old AD&D games would start at a given character level while others would start at a set XP total.
Which required the DM to go through a bunch of extra steps. And I hesitate to call that balance. A Thief might have been 3 levels higher than the Wizard due to XP chart differences, but even a Thief 3 levels higher was not balanced with a Wizard 80% of the time.
For mistakes, if it truly is a mistake that can be fixed, and then the numbers adjusted, sure. What happens, however, when it not an easy fix? Or worse, what happens when it is currently working fine for you, but it gets "fixed" because it isn't working for others, and now has shifted?
Obviously things shouldn't be "fixed" unless they need to be. But "working fine for you" is kind of hard to discuss. For instance, 3e works fine for a lot of people, but it's been my experience that's because their group of people have a certain mindset that says "We will choose not to break this game". Or they have a focus on roleplaying that prevents them from considering the portions of the game that are broken. Or in the case of a group of people I know, they say "Sure you can build all sorts of broken things in 3e. The DM won't allow them, though...so the game is balanced."
While that "works for them"....it doesn't work for me. It brings back memories of friends of mine spending 2 hours on the phone with me trying to convince me to let them take broken options because it fit their backstory.
Which leads me to:
Look, I enjoy well-executed balance in my RPG mechanics as much as anyone. But the idea that even well done balance is consistent across every table is frankly ludicrous. Environment counts for a lot.
It does if people are using the game to play the same way. This is the real problem with D&D. If you balance the game as a way of playing skirmish battles against monsters in a dungeon...it can be balanced well. Until people start using it to simulate a poisoning the wells campaign against a city of kobolds. If you balance the game assuming 4 battles a day it becomes imbalanced to those who play with more or less than that.
D&D is trying to be everything to everyone. The current approach to 5e seems to be balancing between the "3 pillars", so that a class is 80% combat, 20% social...while another class is 100% combat and 0% social. And, although it'll be possible to make the first character DECENT at combat, he'll always be worse than the latter. Which works well if your character gets into social situations. What happens when the DM runs a dungeon adventure that ends when the PCs get to the end? The game will be imbalanced for them.
To me, it just seems that the designers personal tastes lie in more social and exploration....so now the game will focus balance on that. 10 years from now designers that are tired of all the social and exploration will take over and focus the balance back on combat. If your tastes are the same as the current designers, you get a game balanced for you. If not, you wait for another edition. But it's always been like that.
If people are playing the way the game is balanced for then well done balance can be consistent (within a certain range) across the board. Sure, you'll have a class that's a little weaker than another. But when the difference between classes is within a certain small range, it is at least plausible that someone will take the weaker one for reasons other than power.
On the other hand, if you factor in different playstyles, you'll find that a class that can charm people(but only outside of combat) and turn invisible and silent(but not if someone is currently looking at you and it ends when you attack) at-will but who only has a 30% chance of doing 1d6 damage with their attacks will be considered super overpowered in one game and super weak in another game. Those abilities just aren't valued in a game where 80% of the time is spend in open combat with enemies.