The arguable great frustration is that the fluff development in the first two books has been very weak. They've been mainly army updates with a dash of story development. Assuming the remaining books follow the same pattern I can only conclude that it's a bunch of buffs to encourage spending before 9th hits. The sadness here is that the fluff is so limited that the books won't be worth keeping for especially long depending on how significant 8.5/9th ed changes are. Combine the PA books with the Marine supplements and there's a huge potential for backlash depending on redundant all these books are in an edition that was designed to cut down book bloat.
You mean like the Wrath of Magnus and Gathering Storm books? It's not like GW
don't have form for bashing out suppliments when they know there is a new edition in the works.
Having said that, I don't believe 9th edition will be a breaking change the way 8th was. I think it will be a consolidation of all the changes and errata that have accumulated over the last 3 years with a few tweaks that the Devs decide to make. I am pretty certain that the core rules will be basically the same and that all the existing codices and supplements will still be valid. I see PA as a way to give every faction a bit of a mid-life upgrade while we wait to 9th to drop. Whether PA end with galaxy-changing events the way Gathering Storm did remains to be seen. Early indications are that nothing on the same scale is in the works. The books have a dash of fluff progression and some rules to represent that. I think that most of the background advancement is actually happening in the BL
novels (e.g. "Darkness in the Blood" for Mephiston and the Blood Angels). The PA books just give enough of a summary to tie the rules into that.
Sanguinius stood up, stretching his wings to their full extent. He flexed his hands. "I need no blade".
It was as though Sanguinius gleamed with pale light, his face white, eyes becoming blood-red, surrounded by the golden crown of flowing hair. Guilliman had witnessed glimpses of of his brother's wrath before, but had never seen the true Blood Angel unleashed. Sanguinius surged forward on alabaster wings, half a meter from the floor, whiteness streaming from him like flames.