I'm not sure I have any secret sauce to spill.
Bookfunnel does bring me email subscribers. The rate varies by the month. Sometimes, admittedly, it's only one, but it's hard to count, since I know some people who have picked up the reader magnet subscribed later, rather than going directly through the BookFunnel routine. Presumably, they read the magnet first and then subscribed. But I don't have an accurate count for those.
It also performs well in sales, depending on book prices and how well the theme of each promo matches my books. Conversion from clicks to sales is about 2-5% on regular books and more like 5-25% on discounted books, but I have had rates as high as 57% on occasion. Let's just say the ROI is way better than with AMS ads and leave it at that.
It's also a nice way to provide people like my Substack subscribers with book rewards (in that case, book versions of my Substack serials.)
I haven't used it for anything else yet.