I'm still new - just started last year (July) and have done almost no advertising / promotion, and always in KU. I would say that, somehow, readers do find new books on Amazon and in KU. And because, of course, you're a "no risk" option for KU readers, they might give you a try?
I do find an upturn in page reads after a free promotion like Freebooksy, but I've only ever run promotions with at least three books out, so I don't know how it would work with only one book - to be honest, I think you'd really struggle to make any return with just one book, as the real bonus comes in the increasing visibility / rank off the back of the free promo, and the read through to other books.
I started doing some very low-level AMS ads a few months ago and they are helping to keep my books from falling into the million rankings, so for me, they are working - but, again, I'm happy to spend a little on advertising because I get reasonable read-through. (Also, AMS reporting for books in KU does not report borrows, it only reports sales, so my ACOS looks horrendous! But I know I'm making more in page reads, etc, than I'm spending on ads.)
Not sure if that will help, just some random thoughts.