Were the Celts as brutal as the Romans, Saxons, Vikings and Normans when they invaded the British Isles?

At the time of the Roman invasions of Great Brittan they recruited Northern Germanics to fight the Celts because they thought that the Celts were of the same people due to their ferocity and what could be better than to have their own at war with them. The Romans had pre experience with these types and literally didn't want to tangle. Fast forward to Queen Boudicca who fought the Romans and lost however, she damaged the Romans so bad that they abandoned Great Brittan. The Celts were equal to any forces ever and each has had their day.