The sad part is that it probably boils down to they COULDN'T include Radical advancements into the book. You are ascended by the Inquisition, kind of the Inquisition, as a whole, especially if one of you rises to Inquisitor. Now, certain facets of the Inquisition are Radical, while others tolerate them, but the idea that the organization, as a whole, would elevate those who are, more or less, good guys(?) in bad guys clothing, doesn't fly; they'd kill the Radical characters, if they knew what the characters were really up to, so a Radical-flavored Ascension package doesn't seem right; they don't reward Radicals with a package that gives them even more access to Radical stuff. Certainly, you can play one, and if you are stealthy, smart, or well connected, you can get away with it, and it could be one of the most entertaining character types to play, but there's only so much before they see you for what you are in-game, a heretic, or well on your way. It also gets to the point of "how heretic do you need to be, in order to be a heretic?" This isn't a dis to your character, as much as it's a thought of hoe the Council seems to think it works.
I've played most other RPGs worth playing (DnD, WoD, Star wars everything, etc.), and yeah, finding "prestige classes" is often a pain, either requiring some one or two ridiculous requirements, some in a level where you never play till (3.5 Archmage requires a 14th-level wizard/sorcerer, I believe, and many games fall before 14, and DH might do that, too), or they look like something that works in one town, and beyond it, they are nothing. Sometimes you play something that none of them fit with, too. Ascension can kind of do that, also. No, I don't know where some characters should strive to go; there's no super-medic, and the Magos is being played, more or less, by RT characters by Rank 3 (I surmise that the characters are about the same, if there is the L1 RT = L5 DH mechanic, but you feel like you get to Machine much sooner). I would never want to play a DC Assassin, unless I'm going after DC comic characters, nor the Hierophant, Interrogator, Storm Trooper (and that's sad, because Guardsman is one of the base classes I would choose to play), or Sage, while Crusader, Desperado, Judge, and Magos could be fun, but also seem to draw a lot from still being part of their prior organization, which we've already mentioned the Acolyte kind of isn't. This leaves me with Primaris Psyker, Vindicare, and Inquisitor, my three favorite classes, and the ones that are the most reviled, and broken ones of the set. And like prestige classes, some things are beyond you. For instance, I very much like the idea of playing a Penal Legionnaire Guardsman, so he can have a bit more character, and also a few more Skills, but could he get into Desperado? It would seem to fit, but RAW says no. Some of the creative packages they gave seemed to assume that, like epic D&D, "it's a nice idea, but we don't expect you to get here, and actually keep playing." Oh well, I still like Ascension for my bit, and would glady use it, with a few liberal dashes of Golden Rule, and a few addendums other people here have posted. One of the nice things about here is that so many people have vented their frustrations, and so I/we have a good idea of what to watch for, once we get to this stage. I don't expect a reprint of this book; sales would suck of most people couldn't read it, pre-investment, and then why pay for it? Besides, much of their work is probably pointed at Space Marines, the bread and butter of Games Workshop, and Black Crusade, their new baby, who also happens to be a Space Marine. It isn't a great book, as printed, but the idea is nice, and I am more than willing, for my two bits, to play a modified version of it. Hell, I played Space Marine RPG before DH came out, homebrewed from d20, and if that mishmash of suffering and pain could be fun, with the rules changing every session, as our GM kept trying to grind us to one hp (and it still was fun), then Ascension can be, too, so long as the edits are planned out, ahead of time.