Tacitus, Dialogus de Oratoribus 21
“We may grant to Caesar that he achieved less in the way of eloquence than his divine genius would have demanded, largely due to the greatness of his thoughts and occupations. Similarly, we may leave Brutus to his philosophical inquiries. For, in the realm of rhetoric, even his admirers would admit that he was not equal to his own reputation. That is, perhaps, unless there is someone who would read Caesar’s On Behalf of Decius the Samnite or Brutus’ On Behalf of King Deiotarus and other works of the same dull mildness; this reader must also admire the poems of these same men. For indeed, they wrote poems and even deposited them in libraries; though they were no better poets than Cicero, they were more fortunate, because fewer people know about their poetic endeavors.”
concedamus sane C. Caesari, ut propter magnitudinem cogitationum et occupationes rerum minus in eloquentia effecerit, quam divinum eius ingenium postulabat, tam hercule quam Brutum philosophiae suae relinquamus; nam in orationibus minorem esse  fama sua etiam admiratores eius fatentur: nisi forte quisquam aut Caesaris pro Decio Samnite aut Bruti pro Deiotaro rege ceterosque eiusdem lentitudinis ac teporis libros legit, nisi qui et carmina eorundem miratur. fecerunt enim et carmina et in bibliothecas rettulerunt, non melius quam Cicero, sed  felicius, quia illos fecisse pauciores sciunt.