And I'm going to do this in good/bad format...
GOOD: The way Ending E comes out of nowhere, and then recontextualizes a "new game" as, basically, an ephemeral dream Kaine has been having is amazing. Also: holy shit, I was not spoiled on Kaine FINALLY being playable, so the initial "yes you're playing as Kaine" bit was goddamned amazing. Her moves on her menus have swears in them! That seems right!
BAD: It would ruin the surprise, but it would have been nice to be clued into Ending E in SOME context. Like, I only knew it existed (and to aim for it) thanks to that earlier linked review. It is great that you cannot miss it once you initiate Ending D, but "play the game immediately after you finish the game" is about the last thing I ever do.
GOOD: "Ending E" could have easily been just a new cutscene or, like, Kaine walking around some preexisting assets talking to townsfolk or something, but it is definitely its own "game" area. As I was gradually burning out on earning all the NieR endings for like the third time in my life, I seriously considered "just youtubing it" at the prospect of having to replay the earlier areas of NieR all over again (look, I have other things to do, playing through three hours of the "tutorial" section of a game isn't my first choice), but I am supremely glad I did not do that. This is good NieR action, and the gameplay "mesh" sliding into Automata bits was great.
GOOD: Also, the Automata integration is deliberate and obvious, but it also isn't, like, 2-B and 9-S talking about some data file about Kaine they found or something. Like, this is all here from the original text (giant tree is actually a "robotic" memory vault is there in the OG NieR), and the context of it sliding into Automata-like interfaces and graphics fits that perfectly. Also, Yoko Taro loves creepy twin children.
BAD: Also, for a game that revels in its ability to perfectly ape other genres (NieR occasionally becomes a Diablo-like, a Resident Evil-alike, and a text adventure-like as some obvious examples), it's kind of disappointing that the Ending E sequence didn't toss in any deliberate gameplay curveballs. I suppose Kaine being playable at all could make sense, but it reminds me of Automata in a bad way, as I always felt that game was a lot less experimental than its predecessor. Still a great game! But 2-B never had to visit Spencer Mansion...
BAD: (Well, almost neutral.) Those are some GBA remake, let's-slightly-modify-some-sprites level monsters you fight throughout the opening bits of the story. "Slightly modified robots" and "Slightly modified beepy" are... weird choices? Like, they're obviously model swaps of creatures you've fought like sixty times at the point you've achieved Ending E, and I kinda wonder why they even bothered with the new paint jobs to differentiate them. Like, they're just not-different enough to scream of "we didn't feel like making totally new assets". On the other hand, they do deeply remind me of "remake" creatures that have appeared in games like Final Fantasy 4-5-6 Advance, or Kirby Super Star Ultra, so maybe it was deliberate. Maybe "SE remake homage" and "automata homage" are exactly what they were going for...
GOOD: Holy cow is the fanservice on full tilt for the whole experience. "Yonah's Village", Emil's multi-armed return, Kaine literally fighting her past, and the return of the erased vessel. After Weissy got his big hero entrance, I was half expecting Devola & Popola to get some incidental bit of dialogue about secretly helping from the sidelines as an apology. It is a rush to see all this "happy ending" stuff just rapid-firing into your eyeballs.
BAD: Though, that said, on some kind of weird curator-of-games level, I kind of feel bad that Ending D got so damn game-ified now. In the original NieR, Ending D was the ultimate sacrifice: if you wanted to save Kaine's life (genuine question for the player), you would have to delete your save data, and thus obliterate all the progress you had made over the course of hours and hours (the trophy for completing NieR once quickly is on a 15 hour limit, after three endings, you may double that). And it is an action that asked an important question about videogames, as many of us (I am talking about me, at least) complete a game to the best of our ability, and then "save" that perfect save file forever, as if the King of England is going to bust into our basements and demand to see proof that you can catch every fish in Nier's ocean. This is vaguely absurd, but the act of deliberately deleting your own save data is an oddly emotional one, and is appropriately presented as such (watch all your sidequests get deleted!). Now you technically perform the same action... but it's basically just Step 4 of 5 on your way to Ending E. Like, sure, maybe you are coming into that choice "pure", and you don't know about Ending E... but, come on man, we all have the internet. We all have trophy/achievement data, like, right there to tell you you missed something. And it kind of sucks that this almost wholly unique "event" in NieR is now reduced to a stepping stone that can be actively undone if you play the game for a few more hours. Hell, you're literally rewarded for deleting your save data with this new content. Why would you ever not save Kaine now? Doing so unlocks a whole new playable character and weapon.
GOOD: But, again, damned if that Ending E isn't satisfying in a "happy endings all around" way. And the potential for Younger-than-Yonah Kid Nier running around "Yonah's Village"? The fanfic mind boggles.