Vote forward mainly focuses on turnout rather than persuasion. They actually intentionally tell you to not use partisan language in them even though it’s going toward likely dems
Getting dems to show up matters a lot in an election. They do regular testing on it and the letters show a measurable increase in turnout
(And to clarify it’s not based on signups, though they do offer a way to opt out when people receive them)
Overall it increases turnout. They aren’t basing it off of hunches, they actually do a lot of randomized controlled trials
https://votefwd.org/impact
Plus they give a way to opt out from future letters from them if you end up getting one