It's the central problem with AI right now! If this was fixed it wouldn't matter if they were elementary school AIs, they would still be useful if the output was consistent. If they were reliable, then you can find an upper bound on their capabilities and you would instantly know anything below that you can automate with confidence. Right now, they might do certain tasks even at PhD level but there is no guarantee that they won't fail miserably at some completely trivial task.