Just don't read Harrison if you're looking for Heinlein, just as you wouldn't read Delany if you were looking for Asimov, or Le Guin if you were looking for Sanderson.
The End of Everything has ratings that place it in the bottom 0.1% of all published books with >100 reviews. This very strongly suggests that an absolute majority of readers found it frustrating or incomprehensible, and that among those readers are some who had enjoyed his other works.
On Goodreads, that's -4.4 standard deviations.
On Amazon.com, where reviews cluster around 4-4.6 stars, it's -8.5 SD! (Actually quite amazing, when you think about it. It's easily in the bottom 0.1% of titles.)
Check this out: https://emerginginvestigators.org/articles/25-198/pdf
I note that virtually all of the 1 stars are from after it was longlisted. Annoyingly, Goodtime does not provide stats on rating over time.
That is, I've seen plenty of very high ratings on Goodreads/Amazon for things that (in my opinion) range from mediocre to slop.
That tells me there are lots of raters who use some criteria that, while I'm sure it's reasonable to them, doesn't correlate well with anything I'd recognize as "quality".
Glancing at the actual text of the Goodreads reviews, it seems like most of the negative ones are outright saying that they didn't understand the book, and therefore decided that it's meaningless. I'm not saying this because I'm trying to sneer at those reviewers, it's just quite literally what they're saying about the book.
So there are a lot of boos. And even the book's supporters seem to strain to find meaning in it. The most parsimonious explanation is that it's genuinely incoherent.
Not to mention, if you click on show 5 star reviews, it shows the first 10, 3 of which have disclosures about receiving compensation, which seems like a pretty high ratio to me, but i dont know what is normal.