I've had variable luck. I use three different services, and of the three, BookFunnel is the one that produces the most subscribes, followed by Prolific Works, followed by StoryOrigin (which is newer and smaller). For the first two, I use optional opt-in. For SO, the only option is mandatory opt-in.
One reason for the discrepancy, however, is the fact that there are just a lot more options on BookFunnel. I send out my newsletter monthly, on the first of the month, so I need campaigns that start then, which does reduce the list I can draw from. Last month, I couldn't find one on Prolific Works that was a fit for both genre and timeline. Right now, eight future giveaways on PW are open for fantasy and/or science fiction . On BF, it's 26, as well as 31 sales promos, which PW doesn't offer. On SO, it's a little harder to tell, as it uses genre tags and has more mixed genre giveaways, but it appears to have ten newsletter builders that might be relevant, plus some for sales, KU borrows, getting reviews, and getting audiobook reviews. (I've only tried the newsletter and sales ones so far.) In others, PW has action than the other sites. I'd have to do a lot of math to figure out subscription rates, but I think PW's is decent for the promos it has. It's down overall because there just aren't as many options.
It's also worth noting, since the newsletter builder relies on promotion from the participants (plus some push from PW itself) that the number of participants is typically smaller now on PW. My current PW builder has 32 books (and I've gotten three claims). On BF, my current one has 112, and I've gotten 16 claims. So far, only one of those has converted to a newsletter signup, but based on past experience, I'd expect more--the process only started yesterday, so there are probably some who haven't clicked on the confirmation email yet. Anyway, the number of potential subscriptions looks as if it's going to be less on PW, in part because of lower traffic.