Akka's dreams being true is the easily the most verifiable.
The simple answer would be, not even close. Being more elaborate, I can propose several reasons.
Let's take one thing into consideration before everything else. Achamian watches the same events happening differently depending on which dream he sees them in. This alone puts the credibility of his dreams into question. But there is more. He sees things he shouldn't see by definition, considering the nature of Seswatha's Dreams as it's explained by the Mandate. He should see events from Seswatha's perspective, since it's Seswatha's heart he was holding in the Grasping. Yet he now sees things through the eyes of Celmomas and Nau-Cayuti. Why? There is no explanation.
Nayu being a blooded, tormented, toothless wretch, dragged under the IF and put into the Carapace, just like Kellhus is told.
Achamian doesn't see Nau-Cayuti being put into the Carapace. He sees him in the line for it. The Mutilated later confirm that Nau-Cayuti was the first Insertant, but that's it. There is nothing more said about it, no mention of torture that would corroborate Achamian's dream.
You have the Celmommian prophecy through the eyes of Celmommas his self and see how its mistranslated.
This is easily the least credible of Achamian's dreams since it brings no new information, only raising additional questions. It doesn't narrow anything down, it opens the way for so many interpretations the whole exercise becomes counterproductive instantly.
Everything his new dreams are true and we have textual evidence for nearly all of it.
All of those things except the map to Ishual are either conjectures that could've easily been made by the characters without his dreams or just wild inconsistencies that
contradict previously mentioned or recorded Seswatha's dreams and historic facts. The map to Ishual, for example, doesn't contradict anything, so I don't see how it can be viewed in the same light as the dreams that do.
You have to figure it out, put the pieces together.
This is why I'm loath to jump to convenient conclusions. Achamian's dreams being strictly true is exactly this kind of neatly bow-tied resolution that the books themselves warn us against.