I had the Galaxy Nexus and that had a horrible battery and i know the Lg Nexus isn't too good. My S3 has an amazing. I know its not pure google but whats the deal?
Well I know the s3 has a built-in power saver so that helps alot. Pure Google devices don't have this and that does play a part into poor battery life. But what I think plagued the Nexus was that it started off with a new OS (ICS) and it had some battery issues from the start (keep awake bug for those not aware of it). Couple that with a relatively small battery, a big HD Super AMOLED and weak radios, that's a recipe for bad battery life.