Alright. So, first, from the start, the things that I was actually tremendously disappointed with were the pre release things.
The first thing I was not happy with was how they changed E&I to say .45 Art Works. The original inscription said Warks. This was a quirk, sure, but gave the guns a certain personality and it's something that's been part of the canon for this game from the very start. Not a thing anymore (There's a
mod for that, though). They even go out of their way to excuse it in the novel with a line “Guns this beautiful shouldn’t have a spelling mistake on them.” That reasoning didn't sit well with me. It felt like they were treating as if it actually was some sort of mistake.
Then they change the order of the games from 3 then 1 then 4 and finally 2, presumably to follow 5 to 3, 1, 2, 4, 5. Yeah, it's less confusing, for new people, but we got used to it, it's been establish canon for 11 years and everyone who's been with it as long as I know what the score is and it becomes a matter of fact. Hell, the only artbook published to other languages for the franchise is titles 3142.
The other pre-release things that just didn't sit well with me were a whole bunch of things that happened during the prequel novel. The summary description started to get translated and a whole bunch of things just felt awful, mostly things to do with Dante. Right off the bat Dante comes off as a moron who can't remember names of women (and Sparda, too, apparently) because he's so laid back he can't be bothered to. Same with demons. Originally, Dante was supposed to be a man with a superior intellect which is why he was the only one that could stand up to the demons, because they possessed intelligence surpassing that of men. Now he's an idiot who can fight better than most. Grand. It's like he doesn't care or even try to. The man is full of lame dismissive lines and unlikable attributes. The way he brushed off Lucia was so out of character even from what he was like in 2. Yeah, he was stoick but not uncaring. He can't even be bothered to remember the demons he's killed. Aside from fighting, this Dante is a useless wanker and I'd never confuse this sorry tosser for a man of superior intellect.
Then comes the whole Yamato opens gateways, which I didn't make the connection of at first but then it hit me: Yamato doesn't open gates to jack squat. That was DmC thing. Vergil never used it to teleport to anything, ever, until this game and they got it from DmC. Add to that Nero's new look, how they chose a London like city because of their time working with NT and I just knew there'd be more DmC in this game. I don't like DmC. It's not a fanboy thing anymore, I like a lot of the elements it brought to the table, certain decisions in the combat and navigation and I even own a copy but overall I think there are too many issues, namely in the story and the balance so seeing 5 take story cues from it and acting like it was established lore, even though they never belong to DMC, had me scratching at the walls... Metaphorically.
There is this one dude who made a video not long ago, someone posted it here, where he talked about his disappointment with the game. In a segment he talks about how so many elements of the game contradict so much of what happened in the other games that rather than world building DMC5 is world destroying. That's what this felt like, like 5 started to pull apart and take away things that made the game's world, well, it and either replacing them with DmC things or 'correcting' the quirks it had, like a weird release canon or intentional bad spelling of something, as though they were mistakes.
And most of this is just the stuff before the game came out.
PS: Side note: Why does everything take place ten years ago in this novel? The events of the first novel where he got his guns and went by Tony, the Vie de Marli incident, the events of the anime, all take place 10 years prior to this and even Eva died 10 years ago to the events of DMC1 (which they later correct to being 20). Except for the last one they make it seem like everything happened in that one year.