Imperial really has no direct boost on unity (aside from byzantine bureaucracy which everyone get access to, unless you happen to be imperial spiritualist then you are fvked lol) which is really infuriated to me who main imperial.
I'm going to push back on the Imperial Bureaucracy point a bit- I believe people are missing a key point of distinction between it and exalted priesthood, which is the amenity economy and ethics attraction implications.
There's two main roles for Byzantine Bureaucracy- the per-job bonus for its unity, and the planetary bonus implications
The Unity changes to jobs favors the Priesthood. Both civics give their primary base 4 unity job +1, but Priesthood
also gives a ruler pop upgrade from Politician to High Priest, who is both +1 unity (with the civic) and +2 amenities, while coming with a day 1 edict - completely under the Edict Fund limit- for a 20% unity output.
The difference comes from the stability implications, which... also favor spiritualist, especially in wide contexts.
Byzantine Bureaucracy has never been a good 'wide' civic in that you want to put it on every planet to boost every planet. Spending 2 pops to raise an entire planet by 2 stability is not even a 1% gain in output- whatever the planet is specializing in could be better off just hiring another pop. You're more likely to have a bigger stability impact with an Entertainer for excess amenities. The few exceptions to this are the end-game mega-worlds, arcologies and ringworlds, which have the pops for that marginal gain to be worthwhile, but otherwise it's a waste of pops.
In contrast, using the building slot for a temple is a superior option
even if you don't employ a priest, because the building
itself gives a 5% spiritualist ethics attraction modifier. Which means more pops in your state faction, getting happy-faction bonuses and producing faction unity without taking away from the primary focus on the colony, even as that colony has an (admittedly marginal) +1 unity/+2 extra amenities from the High Priest. But what it
also means is keeping pops
out of unhappy factions. A happy faction can give only +10 pop happiness, but unhappy factions can cause -10 or even -40 happiness. Depending on your living standards, these can be
far greater impacts to stability, and a +50 stability swing from unhappy to happy faction is itself a major planetary stability buff far beyond what Bureaucrats could provide, especially when conquests or vassal assimilation is considered.
But what about 'tall' unity-focused planets? Still a strong case for Priesthood superiority.
With the Unity-planet designation, the real role of a Byzantine Bureaucracy is... to stack them on one world, so that the stability bonuses boost stability and get the better overall output. At 100 stability you can get 30% output bonus! Except that this will go down if you focus on the stability instead of amenities, meaning you need to replace at least some of your buildings with Amenity buildings because not only will amenities drop happiness-stability, but negative-amenities will cause ethics divergence, meaning your planet of nothing-but-bureaucrats will decrease your faction unity output if they get too high on their own stability supply.
Priests bring a significant stability with them, without needing entertainers. On top of the spiritualist faction attraction synergy itself giving faction pop-happiness buffs, Priests provide 2 amenity per pop. This isn't enough to be meaningful amenity providers like entertainers, but it
is exactly enough to double their own amenity requirements, which is what you need to maximize stability-through-excess amenities. This is 20% happiness, for up to 20% popular approval on the planet, which is itself 12 stability, or 7.2%.
This is lower, but remember we have a High Priest on
every planet, and Veneration of the Saints for 25% to all priests (and high priests). That's realistically beating the 30% max stability buff.
OR
We could ignore the edict, and consider the impact of a single building replacement from the building slot economy. There's 12 building slots per planet, 1 of which is the capital. Chances are you're using another building slot for pop assembly. This leaves 10. Dropping a single Admin building for an entertainer center isn't
quite a 10% drop in unity output, but it's considerable. Whereas the Exalted Priesthood would replace a Temple with a Citadel of Faith for another high priest, and boost ethics attraction further, etc. etc..
And there's the whole tier 3 building upgrade consideration- I don't believe admin buildings were changed to get it, but temples definitely have it, so a higher-density unity planet which mitigates planet and district sprawl for the same output, thus mitigating tech and unity penalties.
Whether it's wide with a building-on-every-planet, or hyper-concentrated planetary unity production, assimilating conquests or keeping outlier planets stable in a pop-efficient manner, Byzantine Bureaucracy falls behind Exalted Priesthood in several contexts.