Personally I think Advent Loyalists are superior to the Rebels except for Wail. That's a big exception though. Wail is such a great defensive weapon that it makes all the difference. If wail gets nerfed though...the only other Rebel techs I like are the ship clone/ship resurrection ones that give you free ships from owned and enemy ones. Fury of the Unity and Ancient Retribution are both good choices now to buff your late game fleet. I still think Global Unity is too high at tier5 for what it does though...
Imho the main problem with Global Unity is that is basically just a second temple of communion. Imho Global Unity should totally replace the temple of communion for the Advent Loyalist.
More logistic slots for the Advent = better eco
More culture push - fits the factions theme
More dangerous culture as you can can force your enemy to build culture centers in defense without spending money yourself.
Currenty Global Unity suffers from the following:
Harmony Tech 5 - and expensive resarch: By the time I have Harmony 5 for the Advent, I usually will have all the culture I need anyway. 2 temples more are cheaper and more effective than this resarch.
All culture upgrades require the temple of communion. So you have to resarch the temple anyway.
Poor culture rate.... it is cleary designed to be a nice bonus to your culture spread... nothing more nothing less.... but for that I find to way to expensive.
I usually build far more temples for the Rebels for Wail use, than for the Loyalists.
As long as Advent Loyalist culture is only minorly more dangerous than any other culture in the game, as long their faction will suffer.
So to bring the Advent Loyalist to true strength, you need to make their culture more dangerous.... primarly or secondarly.
Examples for secondary effects of culture would be:
A reasonable phase missile block - and not that pathetic excuse both Advent factions have right now. Give the Advent Loyalist a good anti phase missile culture bonus..... and you not only buffed an up faction but also helped the Advent VS Vasari balancing in general.
Stealing ships from your enemy.... perhaps a slight buff to it?