There is an HH novella, it’s not a long read but it’s very good. “The Purge” by Anthony Reynolds, it’s about a Word Bearer named Sor Talgron, who is a soldier first and not really the “incense and iron,” type.
But it’s his internal monologue, regarding Custodes that gets interesting. See, he notes two things, one being that he feels no kinship towards them, despite both being Emperor-made genetic constructs, the second being that even their body language is individualized and opposite Talgron’s men who despite this being pre-Istvaan V, are still in a “pack” mindset. They are not kin to other Custodes, they are bizarrely more human than astartes despite being even further removed from the baseline human.
That right there is why “female Custodes” are more acceptable and minor a change than female marines. I don’t like either, but when you look at it through that lense, I can see it.
That novella also has one of my favourite Dorn scenes, where after Istvaan III and Garro running the blockade, the Imperium is going full Patriot Act and everyone is suspect. Naturally he wants to question why a force of Word Bearers are at Terra, conviniently around this time.
Lorgar, sneaky fucker he was, did not purge a man like Talgron who is not a believer because he was the perfect one to lie to Dorn.
Dorn pretty much does the primarch equivalent of cracking a beer with Talgron and says, “Son, I wish you were one of mine.” After Talgron gives his cover story and expresses embarrassment at his legion’s eccentric behaviour. Dorn proceeds to release the Word Bearer member of the crusader host and lets Talgron and his men go. Which leads to a daemonhost being imbedded in Terra.
A fucking random Word Bearer got one over the Praetorian by appealing to his values. The absolute state of the Imperial Fists.
For anyone who’s read the Word Bearers trilogy, Sor Talgron is the Warmonger, the most badass dreadnought in the setting.