GDPR in no way states that you can't give books to your newsletter subscribers. It states that you can't get unnecessary personal information in exchange for a product or service, and that's what might have tripped people up. Regardless, the legislation is meant to prevent companies from collecting and making use of people's information without their consent. It's not focused on email or mailing lists, like some anti-spam legislations. Bookfunnel has tons of GDPR tools; from stating clearly that people are signing up to your mailing list to defaulting to optional sign-in for Europe. No issues there.
I'll say that nowadays I prefer Bookfunnel. StoryOrigin has some cool group promos as well (and it's free, so why the heck not), and there's always Book Cave Direct (again free, and again why not).
I found out that Instafreebie was really poor at delivering books to subscribers. I built my list mainly with IF, using a novella as magnet. What happened is that every time I sent a newsletter (with a small reminder in case they'd forgotten to download the free novella), the novella got anything from 20 to 60 downloads. 60 downloads! And that's weird, right? Didn't they join my list because of the novella? Or were they just pretending to want the novella to join my list?
Anyways, recently I got my answer. For some 6 months now, I've only participated in giveaways at BookFunnel, StoryOrigin and Book Cave Direct. I rarely get downloads from the novella when I send a newsletter. What does it mean?
BookFunnel readers don't want to read my work. Instafreebie's download process was so complicated that readers weren't actually bothering to get the book they signed up for. Now, this is huge. Remember that now, with the new rules, many of the readers might not even be in your list. You lost them forever... I know people often download books and don't read them, but I'm pretty certain that they'll never read it if they don't download them. So this issue alone makes me favor Bookfunnel.
I'm writing Instafreebie because I don't know if Prolific Works changed anything.
Yes, before, Instafreebie's mailing list was a powerhouse and brought some 1000 downloads to a group promo. I think this is no more, and it's because it has less group promos. I assume readers aren't as interested in the Prolific Works emails.
Another issue I found was that the Bookfunnel group promos in my genres (YA and Fantasy) have better books. By better I mean books from more experienced or bestselling authors, with professional covers. I see less of those in Prolific Works now. And I found that the Bookfunnel promos are getting a lot of downloads, pretty close to what I used to get from Instafreebie in its good days. What I think might be happening is that readers who enjoy freebies are paying more attention to authors' newsletters than to Prolific Works Newsletters, since authors' newsletters have better giveaways.
So I'd say BookFunnel without a question.