Another problem with infinite scroll is often incorrectly implemented "back" functionality in the browser. If you had scrolled down somewhere, click a link or image and then click on "back", you usually do not end up were you were before, but most often on the top of the page where you have to scroll down again to were you were. Very lousy UX.

Even if you solve this correctly by remembering the scroll position, you have to fire X ajax requests to restore the scrolling position, and have to wait until all images are renders or the content will jump wildly around.