Ork fiction in the early codices is just flat out great. There was a real sense of being orky without taking itself too seriously.
I feel like that's gone away in later years. Orks have just gotten bigger, spikier, meaner and typecast. You still get gretchin. You still get squiggs. But they are played straight for the most part. You don't often get a glimpse in to the Ork mindset, and when you do (see: Soul Drinkers series, I forget which book), it's just not convincingly written anymore. Like writers feel the need to explain in plain English how Orks reason, instead of just letting them be Orky and spend time writing that instead of translating for them. Orks don't really talk in the books, other than dropping a line or two to the protagonist. They just roar and grunt and fight for the most part. Even Sandy Mitchell, who has written probably the most tongue in cheek series in 40k novels to date, doesn't really let them shine.
It's a pity. I remember grinning like a fool reading the old Fantasy Ork and Goblin army books.
Part of me wants to say "Yeah but can Orks really hold up an entire novel by themselves?" But then I slap myself and say "Yes, they can if someone actually commits to the style of being orky, to the hilt."
No dropping out of character. No spelling out how orks think or explicitly exploring their reasoning, as almost every 40k novel about humans is wont to do. "Inquisitor BlahBlah ordered his Storm Troopers toward the complex. He knew powerful munitions were housed in an underground bunker there, and if the rebels got their hands on them it would tip the battle for ZippidyDo in their favor. He could not let that happen."