I assume your a child of the early era of 2D mixed with 3D?
No.....
As I said Ive been watching anime since 80s. That's before Ghost in the shell(1995) and cowboy bebop(1998) even existed. There were no such thing as 2D+3D in the 80s anime. At that time it was called "cell animation" which was hand-drawn, which was different from "digital/computer animation" that we know today.
Japanese studios eventually moved to producing anime with computers many years later. That's when 3D technology became possible.
Therefore I wasn't from "early era of 2D mixed with 3D" at all. Nor I understand why people's age is relevant in this discussion anyways, as personal preference on art and media isn't always relevant with age.
When I'm talking anime I'm primarily refering to higher budget movies like ghost in the shell or cowboy bebop if you want a series example
Honestly, I don't think your problem is with the technology, it's the budget.
The reason why you think Arcane and Ghost in the shell looks good is probably because those are some of the highest budget shows of its time.
Regardless of the technology...an animation will look good if the budget is high and vice versa.
If you are going to use shows with the highest budget from that era and use it as a standard, of course lesser budget shows will look worse. There is no way around it.
I've mentioned arcane cause it's one of few examples that I know which did a decent job mixing 3D with 2D,
Just FYI, 2D has always been part of 3D animation pipeline. It's called matte painting. Most if not every show uses it to save money.
It is not a technique that is only used in Arcane. It's used everywhere since long time ago.
The reason why Arcane looks good is because the budget is incredibly high. It isn't because "3D merge with 2D technology" or anything like that.
If you have budget, of course anything can look good. 3D, 2D, whatever. And vice versa.
I've seen plenty of live action shows that I can easily tell which background is matte painting which is real. It's the same "uncanny valley" that you described. So 3D+2D isn't immune to "uncanny valley". It would still looks out of place if it is done cheaply.
I don't know about demon slayer
According to wiki it's the highest grossing film in Japan of all time. Beats Titanic(3rd) and Frozen(4th) by a decent margin.
https://en.m.wikipedia.org/wiki/List_of_highest-grossing_films_in_Japan
So no matter how you think it is cheap or looks bad, it is still insanely profitable, and most people that IS the target audience loved it and paid a lot of money for it. Ufotable's pipeline isn't going away anytime soon no matter what you think.
not hard to find threads or videos about issues with 3D in modern anime
It's true that there are a lot of 3D in lower budget anime that doesn't merge well, that even untrained eyes can see it easily.
But I think 3D in anime is one of these things that when it merged well most people won't notice it, people would pay attention to story and actings instead. When it was done poorly it breaks immersion, and it becomes the target of "3D is bad" mentality.
Therefore it's always easy blame 3D in anime because it only bothers you when you see it.
If you are an artist with trained eyes you can probably see them more easily if you look for flaws on purpose. That doesn't represent average anime viewers though. Most people don't watch anime with a magnifying glass finding every little flaw in the background. They only pay attention to stories and characters unless the flaws in the background are so bad to the point that is immersion breaking. That's when you see people complained about 3D on the internet.