In the process of creating Dianetics, L. Ron Hubbard made many wild claims about the powers of his "therapy." However, the real-world results fall far short of his promises. Let us briefly examine the failure of a few of those claims.
claim #1: dianetics can give a student a perfect memory
(Credit: Wikimedia Commons)
This is a ridiculous assertion, and unsurprisingly, it has not yet been displayed by any Dianetics adherents. It would be easily tested by asking a practitioner what she had for dinner exactly 471 days ago, or having her try to list the names of all the classmates in her second-grade class.
Attempts at demonstrating the claims of perfect memory have not turned out well. There was a particularly embarrassing episode during the early days of Dianetics when Hubbard was lecturing to an audience of 6,000 people in Los Angeles in 1950. He was showing off his so-called “clear,” pseudo-named Sonya Biyanca, to the audience. She had been advertised as having an exceptional memory, but when challenged by the audience to recall specific information, she was unable to answer their questions. In fact, with her back turned to Hubbard, she was unable to remember the color of the tie he was wearing. Nor could she recall what she had eaten for breakfast that morning. 
claim #2: Dianetics will fix vision problems
(Credit: Public domain photo)
Since this type of remark sounds just like a claim of a cure, it's easy to see why Hubbard drew attention from various governmental agencies. Questions were raised about whether he was practicing medicine without a license. 
Nevertheless, Hubbard's grand boasts about correcting eyesight inspired great enthusiasm in his followers. One Dianetics center even readied a fishbowl for collecting any discarded spectacles from those who would no longer need them. However, it apparently never did hold any eyeglasses.  It was just another of Hubbard's promises that never materialized.
“Eyesight, when the stage of glasses is entered (not because of glasses), is deteriorating on the psychosomatic principle... One of the incidental things which happen to a clear is that his eyesight, if it had been bad as an aberree, generally improves markedly, and with some slight attention will recover optimum perception in time.” 
Here we see Dianetics taking real medical knowledge about nearsightedness and throwing it right out the window. Because according to Hubbard, problematic eyesight is not caused by something physically wrong, like an eyeball with a slightly changed shape. Instead, he claims it is just a problem in your head. And guess what, Dianetics can supposedly fix it!
Meanwhile, in the real world, Dianetics and Scientology adherents just keep right on needing glasses and continuing to use them. Ironically, this even included Hubbard himself. The failure of Dianetics to live up to Hubbard’s claims of curing bad eyesight doesn’t phase his followers, even when the proof is sitting right on their own noses.
claim #3: dianetics contains cures for almost everything
(Credit: The Shrinking World of L. Ron Hubbard)
Dianetics gets touted as the cure for just about everything that could be physically wrong with a person. Hubbard also covered the entire spectrum of mental impairments with his announcement that, through Dianetics, he had discovered “the single source of all insanities, psychoses, neuroses, compulsions, repressions and social derangements.” 
Of course, there is no scientific evidence whatsoever to support these claims.    Dianetics has no more footing to stand on than any other product claiming to be a magical cure-all.
the danger of these claims
 Urban, Hugh B. "Scientology, Inc." The Church of Scientology: A History of a New Religion. Princeton: Princeton UP, 2011. Page 61. Print.
 Hubbard, L. Ron. "Eyesight and Glasses." Dianetic Auditor's Bulletin vol. 2, no. 7 (Jan. 1952). Print. http://www.tep-online.info/laku/usa/reli/scien/SECRETDOX/1957_59.PDF#page77
 Reitman, Janet. "The Franchised Faith." Inside Scientology: The Story of America's Most Secretive Religion. Boston: Houghton Mifflin Harcourt, 2011. Pages 60-61. Print.
 Reitman, Janet. "Dianetics." Inside Scientology: The Story of America's Most Secretive Religion. Boston: Houghton Mifflin Harcourt, 2011. Page 35. Print.