I reject most lore advances past 3rd edition for my own sanity.
Different people have different breaking points for how much suspension of disbelief they can have for a setting before it completely collapses on itself. For Louis it's the geopolitics that don't mesh with the "fascist" kill all xenos demeanor that the Iom has become flanderised into.
For me it's eldar making up with dark eldar as if it was no big deal (who cares if you've been raping my family and turning them into drugs or living screaming flesh chairs for generations), primaris marines (0 real impact in lore, exist purely for obvious commercial reasons and any actual interesting plotline that could come from the tensions between OG marines and the new kids on the block is completely ignored) and a billion retcons that on their own wouldn't bother me but put them all together and it's like wtf are you vandalizing your own setting?
In a totally unrelated note, i'm developping a fondness for Marines Malevolent. Why? Well for starters so many people flanderizing them as the biggest dousches around to the point even GW gets in on it is making me like them as a sort of plucky underdog.
But to me they are a callback to RT marines and a nice antidote to space marines becoming "nice guys" as a default rather then exception. I like my heroic space marines don't get me wrong, fond of Blood Angels as the OG cool guy chapter (before salamanders became fleshed into the poster marines for niceness), but when 90% of them are like that it's no longer special.
I fully expect marines malevolent at some point to get killed off in a horrible and humiliating way and millions of plebbit 40k fans will cheer and dance when it happens. Bonus points if they fall to chaos even though a consistent part of the mm lore is they are 100% loyal and never found guilty of any heretical behavior whatsoever despite the many attempts to investigate them, which in itself is impressive.