LilyBLily, I think you have to discount them just like a regular promo to see good results. Or, at least, be in KU.
Something to consider is the audience in these services. Even if a promotion has different authors, I think many of the readers (or authors) participating in these promos tend to be from the same list. So, just like with sale promotions, I think it's good to spread promos out over time. Just like you wouldn't promote your book multiple times in three months on BargainBooksy, I wouldn't try a reader magnet or sales in the same period on BookFunnel. One way to do this is to use StoryOrigin and alternate services (that's what I do, but even these different services, I bet, pull from similar reader groups).
Another thing I do is add information about my books with all my emails (like a cover reveal). I view these promos as a commercial. If you're in my newsletter, you got to put up with them occasionally in order to see goodies about my stuff. But this limits the amount of "free" promos I can run on Bookfunnel.