While this is probably true, it is somewhat curious that C. Cassius (who survived Carrhae) comes off as the hero of the day in accounts from the ancient writers I've read. Couldn't this be a modification of the phrase "History is written by the victors" instead in this case it is written by the survivors? Cassisus being a senior commander in the Roman force would certainly have had the motivation to pin the blame on Crassus (where ultimately as the commander blame does lay).
That being said, even in the battle of Carrhae, with unfavorable terrain, against the supposedly superior Parthian technology, the Roman army stood its ground and battled hard and was eventually able to retreat. There are some inconsistencies in the accounts of the battle. Even though supposedly the Parthian arrows pierced the shields and armor of the Roman soldiers, a large portion of those killed fell after the battle in the uncoordinated retreat during the following nights and days. If the Parthians were so successful why not slaughter everyone at the battle itself? Also despite it being said, if I recall correctly, the roman spears and lances were not able to penetrate the Parthian cataphract's armor, why was the charge of P. Crassus and the Roman "inferior" cavalry so successful initially? You are not forced into retreat by an enemy that can do you no harm. Of course maybe the Parthians realized this because they turned around, encircled and slaughtered P. Crassus and his men.
I think that technically the Parthians would not be invincible to Roman legions, it was a combination of having the advantage of terrain on their side, supplies and the possible mistakes and/or collapse of the Roman command structure. As has been repeated in this thread, subsequent Roman victories over the Parthians with essentially the same forces supports this.