I've always thought that the only reason druids were mentioned by Tacitus and Cassius Dio is that the Roman legions had just had their asses whooped by a girl, while Suetonius was arsing about in Anglesey. I'd imagined his letter home to the boss - "well, yes, we were caught a little with our pants down while Boudicca burned Colchester, but we did invade Anglesey, and they had DRUIDS and everything, and were really scary. As soon as we'd finished killing all the scary people (who sacrifice children, they do, I saw them do it), we went straight back to save London".
So in my mind, druids were just a roman excuse for marching your army to the wrong end of the country while a mass revolt was happening.
sam