I think Pollen's biggest strength is that you can reprogram its markup -- as its documentation puts it, you can attach behavior to tags. I haven't taken much advantage of that yet, but you can do things like create a "TOC" tag that builds a table of contents by inspecting child documents and looking for h1 tags (or looking for, say, "chapter" tags, which you've defined to expand to "<h1 class='chapter'>"), or inspect the contents of paragraphs and subtly shift the first line margin to the left if the first character is a quote mark (which Butterick's Practical Typography does). You could replicate some of that with a template language that allows user-definable tags, but I don't think you could do all of it.
Its biggest weakness, at least for me, has been finding a pleasant workflow. Despite having a built-in web server it feels kind of clunky compared to other static site generators. You're largely on your own for writing a deployment script ("raco pollen clone" is not a valid substitute). The DrRacket IDE is virtually a requirement for Racket programming, but it sucks teabags for editing long prose documents; you'll likely find yourself working in one editor for Racket language files and another for Pollen source. This isn't necessarily a dealbreaker, but it's at the least annoying.