The standard is guilty until proven innocent. Revoke, then appeal the revocation...in which case one must prove the revocation isn't warranted.
There is no presumption of innocence. There is no preponderance of the evidence. There is no beyond reasonable doubt. An administrative law judge may look to "substantial evidence," but only in the appeal process, which is where one first has opportunity to defend himself or herself. After the fact. The firing squad doesn't lock and load until the appeal plays out, but it's still a matter of record, and until the appeal process is done, that revocation order is in place; the pilot can't use his or her certification.
The FAA has a nasty way of taking someone even if they did nothing wrong. A simple administrative letter in the file saying that the airman has been accused of doing X, and that while no evidence was found, X is a violation of the regulation...makes it sound like the airman did it, and got away with it, and simply lacked enough evidence to violate the airman. The airman who is working sees his or her career stalled, sometimes for years, and the FAA moves on to someone else. The FAA knows what it's doing, the personnel involved know they can hurt people, mess with their careers, their lives, even without any evidence at all. I've seen it first hand. More than a few times.
A few years ago a kinder, gentler FAA appeared, seeking compliance and education, but turned out to be the same old FAA, dirty and vindictive, and often made up of those who couldn't make it in the private sector. After all, I don't know of any other job openings in the industry that advertise at such a low bar that the standard is "no more than two aircraft accidents in which the airman was at fault." Good god. Most would end their carers with one...but then they can always go work for the FAA, and I don't care who says the bar is set high. It's not. It's really not. I've seen that play too many times. Say it's not so one more thime, and that's just one more time that the lie is told.
The FAA is not your friend. Not my friend. Anything you say can and will be held against you, and that letter of investigation is nothing more than a fishing expedition to garner evidence to use against you. Period.
Not everyone has six hundred thousand dollars to build a case to save their career, and not all of us can afford F Lee Baily to show up and represent us when the FAA wrongly takes emergency authority for revocation. We've seen this material before.