Like, I know it's a horrible scam that's ruined the lives of many people, but it's done so well I have to applaud.
Why this one scam in particular?
Asking in general, or do you want the OP's opinion?
I was asking him, but if you want to reply with your own take on it, I'd be interested in that too.
I think I've mentioned here before that I was heavily involved in the Anonymous vs. Scientology thing. My initial reasons for getting interested and then diving in had much more to do with Anonymous itself than the target, but as I got deeper into it and fell into the role of propagandist I needed to do my homework on Scientology.
The initial points that brought Anonymous and Scientology into conflict were how seriously Scientology took their bullshit and how vicious they were about shutting up people who tried to expose their bullshit to the public. Critics of Scientology have been subject to insanity that makes conspiracy theorists go "now wait a minute..." and news outlets that have published stories that paint Scientology in a poor light have been sued for literally every sentence of the article. Mormons, JWs, Catholics, any religion you want to point to as a "why not them?" does not do that kind of crap these days, either because they do not have the resources or the will to do so.
The abuses within the church mostly fall on the mentally ill and the members of the Sea Org. Scientologists (like some other religions) don't believe that mental illness is a thing and believe with all their hearts that psychiatry is the devil and psychiatrists are responsible for the holocaust and I am not shitting you. There have been cases of children within the church being denied access to psychiatric care, sometimes resulting in deaths. There is at least one documented case of an adult Scientologist having a mental health crisis and being held against her will, resulting in her death. It's not a unique form of shitty shitty crappy, but it is one more thing to throw on the pile of stuff Scientology gets wrong. The Sea Org is Scientology's private navy that only has one boat and mostly acts as slave labor. People are paid pennies a day, their movements are closely monitored, and they're kept in cramped, unhealthy conditions. If you fuck up in the Sea Org, you're sent into the Rehabilitation Project Force, where you may find yourself inside a locked compound doing manual labor under even worse conditions and little to no contact with the outside world. Members of the Sea Org sign a billion year contract, many of them are minors as young as 12 when they sign up.
Children in Scientology get pretty royally fucked by the experience. Hubbard had some specific ideas about education that sound kind of okay if simplistic at first, but when you see them put into action it's some of the worst teaching methodology you're likely to see anywhere. If a student misunderstands something, it's because they misunderstood one of the words, and they need to go back through the text until they find their MU and look it up in a dictionary. If they get through the whole thing and looked up every word and still don't get it, they have to go make clay models of the concept until they get it. Again, not the worst thing that any religion has ever done, and while the MU and clay models thing is pretty unique, the wholesale omission of math and sciences has a lot in common with whack job Christian homeschooling.
Scientology has the Office of Special Affairs, formerly the Guardian's Office, which is tasked with ruining the lives of people who threaten Scientology and keeping the flock in line. Hubbard and Scientology's love of memos and record keeping means there is a long and incriminating trail of paperwork documenting the creation of the Guardians Office and its transition to OSA and all of the terrible things that OSA is expected to do. I do not know of any other religion that has its own secret police. It's pretty monumentally fucked.
As a money making scheme, Scientology beats every religion running. Every time one of the books is opened in their Orgs, every time they perform a real auditing session (not the stress tests on the street), every course a Scientologist takes, that money is funneled directly to the top. The Ideal Org scheme is particularly odious, as local Scientology communities are expected to purchase large properties, renovate them at their own expense, then open up the Ideal Org and sign the property over to the top brass for free. The money goes to fancy parties for rich people and printing new copies of existing materials that Scientologists will have to buy and redo all their courses for new certificates to keep up, which all costs them more money.
I don't think Scientology is the worst or even most dangerous thing out there right now. I do think the combination of actual physical danger to a minority of their members, significant financial bullshit, and attacking critics like woah makes them a valid target of ridicule and protests.