All of the Mole games run in the past few years, to my knowledge, have been abandoned.
(Blackberry's Toadstool Traitor series has the most famous completed mole games on the site. They had activity problems too, but I think those were mod-side.)
This isn't the only site I've seen that pattern on. I like The Mole and I've thought about running one too, but there are a lot of issues that make running one a lot trickier than something like Survivor.Winning The Mole on the show isn't binary; you have to weigh getting to the end vs. actually getting decent money for winning. On forums, the prize is imaginary, so lots of people will just ignore it entirely and blatantly shit on everything to maximize their chances of getting to the end. And yeah, if you want to only let in people who'll cooperate, then that's your prerogative, but it's sacrificing a major facet of the show*.
So generally you want to make the win condition binary somehow, and then find an incentive for people to work together anyway even though most of them will lose. Usually that means putting in a way for the mole to win if the pot is too low. The trouble is that deciding what's "too low" is kind of hard. Setting the bar is easy when the players are competing against each other, they're doing it for you. In The Mole, you need to figure it out yourself.
Activity's also an issue. Survivor votes out inactive players who can't defend themselves; The Mole doesn't really stop them from coasting as long as they're still handing in their quizzes. And The Mole challenges tend to require all hands on deck--the show can't go on without them so well. Vetting the playerlist helps, but you don't really get failsafes if people flake anyway.
So yeah...I think the issues with The Mole are kind of built into the framework of the game. There are ways to get around some of them, kind of, but they're awkward, and mods need to keep all of that stuff in mind or the game will fail. Sorry for the wall.
*And at that point, you could just run a cooperative challenge that isn't the Mole? Like, you talked about lower player sizes. So why not just put 3-6 players together with one of them as a saboteur and run them through a bunch of challenges with no quizzes eliminations at all? Then tally up their successes, make everyone agree on a ranking of who's most likely to be the mole, and decide whether they all won or lost based on those two conditions. I'm just spitballing here, but honestly this feels way easier to run than The Mole. Hell, someone poke me about this after Team Mafia.