Realistically, though, the amount of "work" to be done by applicant himself is exactly the same across all scenarios, be it application via the embassy directly ($160), via an approved visa center ($198), via an approved visa center by mail ($283), or via "a reputable agency" (the above-mentioned $283 plus whatever fee the agency feels like slapping on top of that).
Only the applicant knows for sure whether they are coming to Russia to "solely, principally, or incidentally engage in prostitution", whether they "have been involved in human trafficking", or "seek to engage in espionage, sabotage, export control violations or other illegal activities". No matter how fancy and/or reputable the visa agency is, they won't be able to give you a list of countries you visited over the past 10 years or give you the maiden name of your maternal grandmother.
Finding and providing all that information is the responsibility of the applicant. The best an agency can do is advise you on making sure the margins are 2mm on the nose (and even then they would be less than truthful, as the application is actually done online, and nobody gives a hoot about margins anyway).