While this is generally true, you can't take the Roman account as an hard-proof, because those accounts were written with a double goal: to depict other populations as barbarians, and to prove that they were good enough to be deserving of the Roman civilization. So the authors described them as bearers of the common traits of the barbarians (lack of laws, practicing obsolete cults that involved human sacrifices, strong individually but unable to coordinate themselves) while also saying that they would fit the empire if they had a chance. You have to keep in mind that it's unlikely that the Kelts were as uncivilised as they result from the Caesar's depiction, for example, since they had, just a century before, a pretty large confederation able to deal with Rome.