Even though it's scary topic, currently they are still in good standing. At least in my eyes. Even though they collect data (how else can you evaluate who to flattr), its stored locally and they do not send it off. If there will ever be a situation where they send something more than they should, this project is dead and there will be no way back.
Looking at their "all-knowing, privacy-friendly algorithm", they are already there:
https://blog.flattr.net/2017/06/key-elements-of-the-new-flat...
Even the title is an oxymoron, let alone the scary description.