I'm not sure about 'idiotics', but the one thing I kind of noticed is how they now referred to the Eldar as the 'Aeldari' or something. If there's something idiotic about the Nu-40k lore, even though this 'retcon' has been there for a few years, is how the Necrons are now somehow in full-control of their selves, and how the Star Gods are now their slaves. It's jarringly out-of-character considering the Necrons are allegedly the only material being that's 'devoid of any kind of life'. But that's just me. My lore knowledge of 40k is extremely limited to Dawn of War 2 games, a single novel about the Iron Warriors (that I luckily encountered in my local bookstores, probably the stars and the planets aligned at the time to make such thing possible) and the wikis. Edit: Oh, and somehow the motherfucking Primarch of the motherfucking Ultramarines, Roboute Guilliman, is back at the stage for some reason. Even though supposedly his throat was slit by Fulgrim using a Chaos-induced poisoned blade that will surely seal his demise, there are zero cure for it and to prevent his death he was put in a stasis, which completely halts the poison progression but in no way can heal him. I'm not sure about others, but for me, another part of 40k's charm is how all these badass characters, either the loyalists or traitors Primarchs, are either missing, dead, or under desperate care to prevent their death but have no means of recovery because their damage is either administered by Chaos, or the means are lost with the whole Imperium stagnating. This means the Imperium are all alone against countless enemies from within, without, and beyond, and then suddenly they brought back the Primarch of the Ultrasmurf.
To be fair, most of these things are easily explained. "Aeldari" have been renamed for legal reasons. Apparently, copyrighting Eldar, Dark Eldar, Imperial Guard etc. is a bit problematic because they are either common SF/fantasy terms or are close to those, hence confusion and legal issues may arise.
Aeldari, Drukhari, Astra Militarum etc. aren't. GW has strong legal grasp on that and they are keeping their products safer. Sounds a bit jarring for old time fans, but is understandable. Most of the people stil refer to them as Eldar so it's not an issue.
Necron lore has been vastly changed in one edition. It's true, that change was really radical and completely scrapped their concept. It's partially because GW wanted to make them a race with more character rather than just soulless killing machines (Tyranids have that covered) and to fit them into a wider narrative of War in Heavens, and make a reverse point or Chaos armies - those not enslaved by gods, but those who killed their own gods. Also, Necrons have later on been set up as great enemies of Chaos and Tyranids, so they are along with Aeldari and Tau kinda sorta potential reluctant allies of the Imperium. Jarring? Yes. Stupid? Well, up for personal interpretation. I myself don't mind and even like Pharaohs in Space and the fact that for the most part they are still, well, dead, tragic and mindless. I understand people who prefer Oldcrons as they are colloquially referred. I should mention that there are Necron dynasties which are pretty much old lore. The fact that dynasties greatly differ among themselves is a good point for me, gives the race more character and variety. And homebrew potential.
I greatly disliked Papa Smurf coming back at first. I thought it was awful for same reasons you mention. However, overtime I grew to appreciate it a bit more. Let me explain.
GW seems to be heading toward End Times more and more. The story has finally progressed toward an even more apocalyptic future - after all, this is the age of Imperium Nihilus. Imperium torn in half, Vigilus is at stake, Cadia had been destroyed and Abaddon is back on Vengeful Spirit, leading Black Legion once more. Daemon Primarchs are running the show. Tyranids are closing in on Segmentum Solar and they are more evolved than ever before. Necrons are waking up. Shit be bad.
Primarchs returning has been kinda sorta announced 10k years ago. They said they would come when the shit hits the fan, and that's about to happen.
RG has been the one to return for two reason.
Meta reason is that Ultramarines are rhe single most popular faction on the table.
However, setting wise he is the most logical choice.
He is the only Primarch whose whereabouts are actually known. He is there, in stasis. And getting him back recquired the assistence of Eldar god of death - an actual fucking god. No small feat. Plus, he is the most diplomatic and pragmatic Primarch - something Eldar needed. Some of the stuff were poorly handled (Primaris Marines) but RG makes sense. And he (and Ultras) are actually cool now. Gone are the days of Wardian Heresy...for the most part.
Plus, more Primarchs are coming. My bet is on Lion being the next.
WH40k universe is slowly coming to an end. It will take a fuckton of time for that to happen in real life, decades at least, but in-universe? The end is nigh. Expect more (Daemon) Primarchs and more insane shit going down.