I have 4 encounters left to complete at lvl5, and for the reference - the military boost of the Inno army for encounter 77 is 527/527.
Not going to go further, because do not see incentives to do so. And probably will do, may be, 4 lvl5 encounters next session.
Lvl 5 reward buildings are not very different from good event buildings, but building these lvl5 buildings and leveling them up will take a lot of time, and require either tons and tons of goods, or boosting your defense a lot. I have better use for my goods, and do not see the point of boosting my defense because it makes use only in GE lvl5, and is complete waste of time and cite space therefore.
Temple is temporary, and I do not like this kind of buildings.
Feathered Serpent looks stronger than Sentinel Outpost and Nutcracker Guardhouse, but will take at least 3 months (if you are lucky) and a lot of resources to level up. And based on direction of the recent game development, we will probably see even better event buildings before we will have any serpents fully leveled.