I personally found the ending as disappointing as the rest of the book, and just as rushed.
A bunch of Titans turned up from nowhere to rescue some Ultramarines and extend their survival, only if they hadn't, the UMs would still have lived long enough to gain control of the planetary defences. So the Deus Ex Machina of the Titans didn't even achieve anything, it just happened.
Having taken control of the planetary defences, some of the Word Bearer positions were bombarded. Only they would have had to go underground due to the sun blowing up within a few hours anyway (leading me to wonder why the WBs stayed on the surface for so long - and why blowing up suns doesn't happen more often, instead of Exterminatus). Granted some WB ships were destroyed... which I suppose demonstrated the WBs didn't have everything their own way and the UMs aren't rubbish... but it just left me thinking "so what?" It wouldn't have made much difference if it hadn't happened.
Kor Phaeron didn't kill Guilliman when he could have, but I guess that's standard muahahaha bad guy behaviour; in fact, I think if there had been time and space in the book to really explore what happened between them (instead of a paragraph or two saying "Kor Phaeron decided to try to turn Guilliman" and a few paragraphs in which Guilliman basically said "no thanks") this might have been quite interesting; even if we assume that nothing could feasibly have tempted Guilliman even for a moment, it would have been enlightening for Kor Phaeron's character to see what arguments he might have tried. As it was it was all over before it had even begun. So Guilliman got a chance to flex his muscles but again, it left me wondering "so what?"
There was another massive "so what?" with Oll. He's one of John Grammaticus' colleagues, he's got serious plot armour and he "just knew" exactly which point on the planet's surface (which happened to be nearby) to go to, and what to do once he got there, to escape the planet. But... he didn't do anything. The entire Oll thread could have cut from the book and there would have been zero detrimental value whatsoever. One assumes that he'll turn up in a later book at his new destination and coming from Calth will turn out to be important, somehow. But it's hard to imagine how. He was a civilian on Calth but in the Cabal; he could as easily have started at his destination (which at first I took to be Colchis but re-reading I'm unsure) without having come from Calth.
 Almost forgot - then there was the Dreadnought! That story started out with potential (it's rare to get inside the head of a Dread)... then... just... stopped. Nothing happened. Again I can only presume it was the lead up to another book, but it could have been removed entirely from this one. [/edit]
The biggest "so what?" of all... "It is the beginning of one of the most infamous naval duels in Imperial history." Wait. That's it? Really? I can only assume, again, that another book will reveal what happened there, but by then all the context will have been lost. If another book does pick it up, it's a really strange place to cut it off. If it doesn't... WTF?
It's possible that other books will fill in the gaps but standing on it's own, Know No Fear should IMO have cut out half of what it contains, and been twice as long, replacing all the cinematic set pieces with the characterisation and dramatic tension that Abnett normally does so well. Like I said before - if you want action, it's here by the bucketload, and I suspect a lot of people will love this book for that reason (and it is, after all, a book about a battle; perhaps it is unfair of me to expect more). But if you want dramatic tension, character development and a cerebral plot, this book, IMO, and unusually for Abnett, does not deliver.