I'm agree with you but I gotta say, I have yet to find the perfect tutorial that say "follow these N steps and you'll always get what you expect".

For me, the common places things break are, setting up a flexbox and not having the content scale to fit.

Trying to make an app like layout (so the page itself doesn't scroll though some content in various areas might) and again, trying to get various flexbox areas to either fit their content or get the content to fit its area without expanding the page past the window size (note: if you use overflow: hidden you failed)

In any case I'm not a UI/UX designer so I don't do it daily, therefore when I do need to do it it's always a bunch of frustrating trial and error.

That said, I do think there are a class of designers that also don't know these answers. They just know a few frameworks that handle all of this for them.

It is a sunk cost for them. They literally cannot get it back. But for somebody new why should they have to pay this cost if there are many less expensive ways to describe a UI. The lowest common denominator is now invading everything including native. This often happens but it is regrettable.