The only problem with Flattr is that it doesn't work.

Other than that it's great.

I mean this both straight and a bit sarcastically. I'm ideologically inclined to want to see it work. But it doesn't, it hasn't, and I see little reason to expect that to change in the future. Patreon doesn't work perfectly, and perhaps there's an even better model waiting for someone to find it, but it does work.

It does strike me as the sort of thing that would be a decent VC candidate. It is plausible to me that Flattr's core problem is an activation-energy one. But I can't run that experiment myself.