It's stuff like e.g. the interface between the receiver and the buffer tube, which is weak and known to break under stress. Or the telescoping stock design, which has a similar problem with the retaining pin - which is why they tell you to never mortar it unless fully collapsed.
There are some issues with the bolt longevity - in an AR, the bolt eventually fails with sheared locking lugs - which is unheard of in AK and not common in other designs. Part of it is due to asymmetric distribution of forces, because of the extractor taking the place of one lug. Modern bolt redesigns try to mitigate this in various ways.
Then there's the enclosed magwell, which is great for quick mag changes, but requires the upper part of the magazine to be straight - which caused troubles for 30-round AR mags historically, until the modern polymer magazine design with internal constant curvature made possible by the manufacturing process. In addition, the magwell is prone to dirt-induced malfunctions - once you get the dirt in, e.g. from the lips of a magazine that fell to the ground, it stays inside, and can block the mag from going all the way in easily. Ice can cause similar issues. The button magazine release can get stuck relatively easily as well. AK well-less paddle system, while slower, is less susceptible to dirt, and the rocking insertion motion and the paddle magazine release provide more leverage even if there is an obstruction.
Or we could look at the antiquated handguard design, which results in precious little rail space on the upper receiver, because the top rail on the handguard, even free-floated, is not as stable (and hence not used for magnifying optics usually, but red dots, lasers etc). Notice how the latest trends in military ARs have been monolithic upper receivers, like in New Zealand's MARS-L, or Canadian MMR?
Of course, you could easily make a rifle based on the Stoner action that has none of these problems - there are more exotic AR variants and accessories that solve one or the other, you'd just need to combine all those solutions. But I don't think it would count as an AR at that point. Even the monolithic upper is already a major difference that breaks compatibility in so many ways. I mean, we don't say that SIG 550 is an AK variant, even though it's also long stroke piston, and the BCG looks very similar to AK. There's more to a gun than its action.