The usual CSS generated content can combine different chunks of text and data, such as :

a::after {
    content: attr(href) ' — ' attr(data-whatever);
}

Paged.js custom generated content cannot combine chunks like a counter and a element:

@page {
    @bottom-left {
        content: counter(page) ' — ' element(runningTitle);
    }
}

If an element is within the content: rule, it overwrites any other content.

An element can contain html (which is nice) but adding generated content text nodes next to it could be smoother as it resembles more usual CSS behaviour.