Rick noticed something yesterday that I had noticed as well: our installs of Feed on Feeds don’t grab all comments in WordPress’s comments feeds.
I looked at the feeds, and I think I know what the issue may be: the feeds are done in chronological order, rather than reverse-chronological—that is, the comments are presented oldest-to-newest, rather than newest-to-oldest.
For an example: check the comments feed for “Yay for Todd and Sarah”. There are, at present, 16 comments to that entry; the feed only lists the first 10 comments.
The 10 is significant because that’s how many items I have set WP to publish at any one time in its feeds.
Even if chronological order makes sense for the publishing of comment feeds—and I’m not sure that it is!—doing so breaks functionality anytime you have more comments than the value in posts_per_rss in the wp_options table.