Original sentence: More than a century of criticism, argument and counter-argument have argued that the Augustan History is not a reliable source for  the period that it purports to cover, and it is especially unreliable in that era for which it is one of the very few written sources, the years 253–284
Edited sentence: More than a century of criticism, argument and counter-argument has established that the Augustan History is not a reliable source for any of the period that it purports to cover, and it is especially unreliable in that era for which it is one of the very few written sources, the years 253–284