I think the lack of wider scale meshnets is just that the people willing to set one up or join one are either geographically isolated or aren't aware of others in a similar position. You need a high density of willing participants for something like it to work. Yes there's issues with authentication and flood control, but I'm not sure meshnets have ever become a big or important enough thing for that to be an issue (I'd love to be corrected on that).