<header> and headings are very different. A heading is how you semantically signpost the beginning of a different section of content. A <header> other than one at the beginning of a page, is basically a DX convenience.
Headings are the major way people using assistive technologies can skim a page and navigate around. I think a <header> will still be treated as a landmark, even if it's inside other landmark elements, but it will be nameless so not nearly as useful.
It can be a struggle for some people in some situations to create pages with a heading level hierarchy that accurately reflects the content. Sometimes I think when there's any difficulty, authors should basically give up and follow a simple guideline: have one <h1> that says what the page is about and use <h2> for all the other headings.
The words put inside the headings are far more important than the accuracy of the heading level.
Headings are the major way people using assistive technologies can skim a page and navigate around. I think a <header> will still be treated as a landmark, even if it's inside other landmark elements, but it will be nameless so not nearly as useful.
It can be a struggle for some people in some situations to create pages with a heading level hierarchy that accurately reflects the content. Sometimes I think when there's any difficulty, authors should basically give up and follow a simple guideline: have one <h1> that says what the page is about and use <h2> for all the other headings.
The words put inside the headings are far more important than the accuracy of the heading level.