If you came out of this novel going... "I could really play the Iron Warriors," and especially if you've read about the other Legions in the heresy, boy do I feel bad for you.
I'm not sure that was the point of the book.
But let me ask you this first. What would make you think "I could really play Iron Warriors"? Because if it's scenes that expand on their reputation as siegesmiths, then there's a lot of stuff that does that already...
I think Graham made a perfectly conscious decision in making them feel like any generic space marine legion. Granted, it might not come across as a good thing, but that's not my point. He's specifically avoided getting into siegecraft for this novel (although the man can write a good siege, as Storm of Iron proves) for a reason.
Even you, as a fan, expect them to fit a certain style that you think suits them. Here's the funny thing. In universe, everyone expects them to fill their one specific role, as well. Everybody just assumes they're the Emperor's diggers, and their frustration over repeatedly being cast into that role is actually a considerable (although not the main) part of the reason Perturabo renounces his oaths of loyalty in the first place. It's also an important factor in his decision to play along with Fulgrim's folly of a plan.
So, now, uncharacteristically
and for the first time, we see them take to the field as a mobile fighting force, and they fare no better or worse thany any other Astartes legion. That's the point Graham is (granted, not all that skilfully) trying to get across. The Iron Warriors aren't all about sieges and trenches, they just happen to end up dragged into them every time.
I sort of liked the fact that they can fight coherently in the open, and even put on a better show than the Emperor's Children. And I sort of liked that even then, some of their smaller quirks still surface (i.e modular Rhinos, use of field artillery, etc).
Could it have been done better? Certainly. The irony of the situation could have been capitalized on a lot more, and more memorable fighting scenes surely couldn't have hurt the book. But they didn't come off as badly as you make them appear (or, more to the point, as you see the whole situation).
I do find it somewhat unfair that you're judging the portrayal of the Iron Warriors bad, when the whole point of that portrayal is to cast away some of the (unreasonable) stereotyping that's been built around them over the years and in the absence of any half decent background fleshing (Storm of Iron notwithstanding). But, I guess that depends on the question I raised at the start of this post. What would make you think "I could really play Iron Warriors"?
If you wanted to see Iron Warriors do what they do best, there's going to be ample opportunities for that later on. The Siege of Terra. The Iron Cage. Tallarn, quite possibly. But just not now. This was Perturabo's book, and a lot of what happens in it, happens because of his decisions, the choices that reflect his character, simple as that...
If you wanted them to truly shine in a situation that doesn't really become them, though... well... I suppose then you can blame Graham for disappointing, but if had he done that, it would only have served to make some other legion less special in their own book...