I think Spacebar has it right:
Nixon’s trial was imminent. What Ford basically did with his impressive document (and unconstitutional imho) on White House letterhead was imply that he’d be pardoned in any event since Ford would still be President so don’t waste your time, and no one ever called his bluff in a court of law or in Congress. Instant precedent, just add water.
If challenged it would not hold up. But it was not challenged.
Ford’s pardon of Nixon was challenged at least once and thrown out of court.
Supreme Court decision in Ex Parte Garland (1868), citing common law and original intent, has never been successfully challenged and isn’t likely to be.
There have been a number of presidential pardons/amnesties with no convictions.