Harzel
Adventurer
Sure there are. Monks’ Unarmored Defense, for example, doesn’t stack with armor or shields.
Um, sort of? It's not so much that it doesn't stack with armor or shields - you can't use it at all if you are wearing armor or using a shield. So, for example, if a monk with 16 DEX and 14 WIS puts on leather armor, they cannot use UD and their AC actually decreases from 15 to 14. So having armor or a shield gets in the way of using the defensive techniques you learned in monk training. That's a lot different than trying to explain Barkskin.
Barbarian Unarmored Defense is likewise inoperative when wearing armor. Armor gets in the way of barbarian training, but a shield doesn't. Eh, ok.
A tortle’s natural armor doesn’t stack with manufactured armor.
An explanation is provided.
Tortle said:Due to your shell and the shape of your body, you are ill-suited to wearing armor.
Now of course that would make you think that you might be able to pay a smith enough to make you some awesome tortle-shaped platemail. But, RAW, you'd be disappointed.
Tortle said:You gain no benefit from wearing armor
Yeah, not supported very well by the fluff, but again it's also not really like Barkskin in that it's a blanket prohibition. Something about tortleness inhibits the effectiveness of armor. Period. It's just a base AC calculation that you cannot use. It's not weirdly situational and all AC bonuses work as expected.
Mage Armor is another one that just doesn't work if you are wearing armor - again, not a stacking issue.
There are several instances where you must choose the higher of two armor sources.
But are there really? Despite the note in the PH that if you have more than one way to calculate AC, you get to choose which to use, none of examples above actually result in a dynamic choice, that is, a choice between two AC calculations that are operative simultaneously. It is very possible that I'm forgetting some. (?)
Barkskin happens to be the only one that doesn’t stack with any other armor source,
Yes, and, in fact, I realized while writing the stuff above that it is the not-stacking-with-bonuses for reasons that are either hard or impossible, depending on your take, to explain in the fiction that is, I think, the most unsettling. The most illustrative example I can think of is cover. Whether to seek cover is a fairly common choice facing players during combat, and it's not uncommon for there to be a trade-off between seeking cover and some other move that is gives a different advantage. One (or at least, I) would like that decision to be able to be made largely by just engaging the fiction. If Barkskin is in play, the player will have to engage the rules just to see if cover will, qualitatively, be any benefit at all.
So maybe it is a little bit more nuanced (and, maybe, arguably, pickier) that just "this is hard to justify in the fiction". It's the kind of situation and decisions that are affected. Other situations that I can think of are less dynamic than cover, but it's still weird to have to do arithmetic to figure out whether an AC bonus will do you any good at all.
and is also the only one that represents a source of protection that doesn’t involve an element of the difficulty of hitting the creature.
Yes, which makes me wonder if there were more instances in which such a mechanic were used, if it would be less confusing/objectionable. That wouldn't exactly be rational, but it could be understandable.