The thing is, that each building has two sets of buffs, one which are available each time the building is up (hi mage tower!), and the other being random among several per building (3-4 i think) - they are announced when the building is begun building, and sadly, will not change (like now, nethershards pff) - im not entirely sure if they show up in-game, however wowhead and the companion app list them, so you can be prepared! (illidan approves)
This :) Nethershard buff this time!
You can see all of the buffs for each building and how they work on Wowhead's guide to the Broken Shore
here.