It isn't harder to maintain at all (define maintain). It's the same as guilds except that you need to invite others first before they can join. Even if you know where it was even the region lord can't close it down as he can't see it if he's not a member. Increasing membership is also not necessarily a goal for secret societies. If everyone was a member it wouldn't be secret anymore.
And my point was on the supposed internal secrecy or the lack of. I suppose though if members didn't know one another it would turn into the underground.
