Tell it to Crassus, Valerian, and Julian the Apostate.
In their prime, though, the Roman Army was unequaled. I remember as a kid reading about Hannibal’s great campaigns. I thought he was the greatest general of ancient times. He may have been, but what impresses me now is how the Romans stayed the course after suffering the loss of several complete armies and crushed Hannibal and Carthage. Also, the campaign against the Jews was impressive. They were fighting a foe that fought to the last man, but the Romans ground them down. That’s how I think of the Roman Military Machine at its height - like a huge grinder that nothing could escape.