I feel like bounded accuracy - at least as it's implemented in 5e - makes no sense in several ways.

Which I know is a bold thing to say, considering that it's one of the most important design tenets of the edition.

But as much as the promise of it sounds good - the DM's enemy selection possibilities only ever expand, the world can remain more consistent, it cuts down on math, less specialized characters can still be useful, etc. - I feel like there are quite a few things about it that - at least in my opinion - just... don't stand up to scrutiny.

• Several things that really should scale instead don't. At all.

Say you have a level 1 Fighter in plate armor. Yes, I know a level 1 Fighter is unlikely to be wearing plate armor, but humor me for a moment. That's an AC of 18. So if a random commoner (+2 to hit) takes a swing at this Fighter, they need a 16 or above to hit - a 1 in 4 chance.

That makes sense. Plate armor is incredibly protective, and it'd take a truly good hit to do any real damage, so it makes sense that it's a low chance. But at the same time, this is still only a level 1 Fighter - still probably a skilled warrior, but not a master yet. They're probably not defending themselves perfectly. So it makes sense that even a random commoner has a decent chance to deal damage.

Now say you have a level 20 Fighter in plate armor. The greatest warrior in a generation, if not in several; slayer of great fiends and ancient dragons; an adventurer who's been through things no ordinary person can even imagine.

Now if a commoner takes a swing at that Fighter, what's their chance to hit?

1 in 4.

Yes, apparently in all those 20 levels of adventuring, amidst ancient dungeons, facing down ferocious monsters... a Fighter has not learned to use their armor to even slightly better effect than at the very start of their career.

(Of course, in practice it's true that a Fighter's AC will improve due to upgrading to plate armor from a weaker type, and possibly due to magic armor. But that's due to their equipment, not due to any of their own skill.)

Light armor wearers, who rely on Dex for AC, have a somewhat more reasonable progression. But even then, you're likely to hit your Dex cap at level 8, which means that for the remaining 12 levels of adventuring - over half your adventuring career - you apparently, again, don't get even slightly better at getting out of the way of danger.

There exists a similar issue with saving throws. I know the common complaint with saving throws is how much they screw over martials, and how much having a bad Wis save at high levels effectively just disables you instantly, and I share those complaints... but it's hardly as if this only applies to martials. Say a level 1 Wizard starts out with, like, +2 Dex. By level 20, they maybe have, at most, +4, and they're very unlikely to have gained prof in Dex saves. So over 20 levels of adventuring, going from a somewhat skilled caster fresh out of academia to a legendary, battle-tested adventurer with dozens of fights under their belt... this wizard has barely gotten better at getting out of the way of things.

(You may say that this is fine because D&D isn't meant to be a world simulator, and it's fine if some things aren't realistic. I agree. But in this case, it's a form of being unrealistic that also works against the feeling of characters progressing and becoming more skilled and powerful.)

• When things do scale, the final numbers they obtain - barring features like Expertise - seem disappointing for a supposed master.

Let's go back to that level 20 Wizard. An absolute master of the arcane arts, the mightiest and most knowledgeable wizard in all the land - almost a demigod, capable of Wishing reality to be as they desire.

Now, barring anything like Expertise, and assuming they have prof in Arcana, their Arcana is +11.

How good is that?

Well, first off, a "very hard" DC is 25. So this legendary wizard, with such magical knowledge that they're capable of warping reality, resurrecting the dead (Wish for Resurrection), creating vast illusions, and causing incredible destruction... cannot consistently pass a "very hard" Arcana check.

But it gets worse.

Say this wizard and a random commoner both roll an Arcana check.

Let's do some math.

The commoner has +0 Arcana, so the commoner will never beat the wizard's roll if the wizard gets a total of 20 or above. So for the commoner to have a chance to beat the wizard's roll, the wizard must roll an 8 or below (8 + 11 = 19).

Now, the wizard has a 1/20 chance to roll any individual number. If the wizard rolls an 8 (total 19), then the commoner must roll a 20 to beat that roll (a 1/20 chance). If the wizard rolls a 7 (total 18), then the commoner must roll a 19 or a 20 (a 2/20 chance). For a 6 (total 17), it's 18, 19, or 20 (a 3/20 chance)... and so on.

All in all, the commoner's chance of doing better than the wizard on an Arcana check works out to:

(1/20)(1/20) + (1/20)(2/20) + (1/20)(3/20) + (1/20)(4/20) + (1/20)(5/20) + (1/20)(6/20) + (1/20)(7/20) + (1/20)(8/20)

Which WolframAlpha says works out to 9/100.

...I should note, that's very nearly 10%.

Look. Maybe by some freak accident, sometimes it's possible that the commoner knows something about magic even a legendary wizard doesn't. Perhaps it's an ancient secret passed down through their family, or an artifact that they just stumbled onto and somehow managed to figure out by dumb luck. It's possible.

But it should not be happening nearly 10% of the time.

• It seems to drag the tone of the game in a different direction compared to... everything else.

This is sort of connected to the previous one. But in any case - let's consider what the principle of bounded accuracy means. At least as it's implemented in 5e.

It means that the abilities of PCs will always be - at least somewhat - reasonable compared to that of ordinary people. Of course, even in a system with bounded accuracy, I presume a level 20 character is meant to be very good at what they do - but they're not inhuman. You don't have untouchable warriors that no ordinary soldier can even scratch, nor do you have wizards with impossible knowledge that can casually tell you the deepest secrets of the universe. In this universe, even the greatest among ordinary mortals aren't on the level of mythic heroes or legendary demigods - they may be extremely good at what they do... but ultimately, they're just mortals. It's a more "grounded" take on fantasy.

This isn't a bad thing.

What is a bad thing is that, as you probably know, basically the entire rest of 5e's design pulls it in a totally different direction.

If you look at the DMG, this is what it has to say about tier 4.

By 17th level, characters have superheroic capabilities, and their deeds and adventures are the stuff of legend. Ordinary people can hardly dream of such heights of power — or such terrible dangers.

Well, that already certainly doesn't sound like what I described.

And if we look at the Monster Manual - what are some monsters of CR 15-20? Well, adult dragons, for one, and I feel like I can essentially stop there. But it also includes powerful fiends - balors, pit fiends, and Bael, an actual archdevil. You also have gargantuan monsters such as frost worms, hundred-handed giants, the elemental princes... certainly not the sorts of creatures you would expect the heroes of a "grounded" story to be able to face and defeat with regularity, even at the height of their power.

And of course, then there are spells. One look at the available high-level spells should tell you for a fact that there is no way spells like Wish, Resurrection, Time Stop, Clone, Imprisonment, Simulacrum, Teleport, etc. were intended for a "grounded" game with protagonists that remain within "human" levels of power. HP is another thing to consider - it's been mentioned quite a few times on this subreddit that many higher-level characters can survive falls from any height, and in general, high level characters can endure some things that should be clearly lethal to ordinary humans, even highly skilled ones.

Overall, there are many, many aspects of 5e that pull it in the direction of being a game that is, at high levels, about legendary heroes, easily equals to the figures of myth - the descriptions of the tiers, spells, the monsters you fight, etc. Essentially only bounded accuracy (and to a certain degree, class feature design) pulls it in the other way, in the direction of a far more grounded system - but because it's such an important principle in the edition, it pulls hard. The end result of that, it feels like, is to make the high levels weirdly incongruous - you're casting almighty spells, casually fighting beings straight out of myth... but your "mundane" skills are, like, just pretty good at best.

(Expertise does help with both this point and the previous one, but for a long time that wasn't even available to many classes without multiclassing - and even now you have to spend a feat to get it. As you might infer from what I've said here, I'd much prefer it to be a baseline thing everyone gets in at least some fashion - though honestly, I'm not even sure that'd be enough.)