"Fancy" scrolling often breaks my favorite way to scroll: by clicking the middle mouse button and just dragging the mouse relative to the anchor point to control the scroll speed. I'd advise anyone who wants people to stay on their page to just leave scrolling alone, but if you're gonna mess with it anyway, at least consider all of the possible modes of scrolling people use and how your new take on scrolling will affect their experience on the site.
It’s impossible to reimplement scrolling in any way acceptably on the web, because the web doesn’t expose several of the primitives you’d need. Therefore I say: scrolljacking is always bad.