Adell

Ignoring Word of God (because fuck 'em, unless they're at my gaming table, they don't get a say except in what published stuff I use), the setting material paints a pretty clear picture to me of a well intentioned bureaucracy that suffered a fascistic takeover by militant chauvinists, then that bunch were deposed but the process of replacing and remaking the infrastructure of empire mostly laid out by SecComm is incomplete. Which is the main reason in the here and now for allowing the corpro-states to do their things: at least they're their own beasts and not some local Union administration post that, well, sure they heard on the omninet about changes, but it's still seccomm loyalists on the ground running things. Or some battlegroup that just came back from a clash with the Aunic fleet, some objective hundreds of years ago, and still hasn't ever internalized the change of doctrine. But is embarrassingly loyal and strict in following orders - it's just they're so senior and the way they interpret conflicts is so aggressive. Etc. Union is mainly in conflict with itself. When I hear "Union is benevolent" I interpret it as "there are a lot of people working for the cause of Union who truly believe in it and are fighting to make that real."

I hope that perspective is a little more on point than whoever gave the disappointing response. In short, I view the inherent struggle to make Union real, despite its contradictions, as one of the more interesting things making it possible to actually play Lancer.

There's technically precedents but lots of them are complex or need digging into other modules(or even outright drafts of one).
Plus evolving lore and all.

I think a good example, that's sadly only really fleshed out in their most recent draft(that's currently on hold due to Miguel Lopez getting hired and then shackled with a non-compete by Wizard of the Coast) is the Aunic Ascendancy, which is even actively at war with Union over that disagreement;

While their earliest draft portrayed them as ridiculously violent crusaders types who were originally designed as a threat who had plans to go out and conquer Cradle(something not impossible considering they feature one of the most notable example of a non-Blink based FTL that doesn't depend on massive space stations).... their last draft before Miguel got nabbed might portray them closer to what you're talking about.

Auns are notably because they literally were one of the people that left during the Fall on a slow generation ship toward a new promised world.... only to find that much faster and recently built Firstcom-era nearlighter managed to get there first and had a colony running for an hundred year already.

After being refused entry(due to quarantine/triage/etc protocols) they essentially had to menace the colony to de-orbit their ship on it for them to finally be allowed to land by which point they essentially assimilated the smaller colony over time(this was pre-Blink era so the colony didn't really have contact with Union).

They'd develop for a bit until Firstcom would expand again in their neighborhood which they discovered about when social troubles and revolutions(which Firstcom couldn't do much about as this was the pre-Blink era) led them to investigate what was going on and ended up taking up some of the Union worlds while others remained indepedent; the whole region eventually becoming known as "Boundary Garden" and is essentially notable as the 'border' between Union and the Aun.

Then Firstcom hearing the news was essentially overthrown by the radicals that would go to immediatly form SecCom as they answered by essentially shooting an interstellar kinetic kill strike, PISTON-1, that would essentially destroy Aun'ist millenias from then once it'd reach the world(it still hasn't reached the world as of 5016u and is, in fact, considered strangely 'missing', which some is believed to be the result of intervention by the MONIST entity worshipped by the Aun, Metat Aun, but the unknown status of PISTON-1 is cause of great anxiety amongst both the Aun and Thirdcom Union; in fact late SecCom era's unwillingness to do anything about the acts of the SecCom early founders was one of the many motivations behind the ThirdCom revolution iirc).

But the important bit for you might be how though yventually things would settle down in the millenia afterward with the Aun even trading indirectly with Union through boundary.... they never formally joined or even were interested at all to join Union ever;
They still recalled how it had almost taken their promised world by getting there first without asking if it was meant to be destined for others(after all, they came from Cradle and believed people of Cradle should have known), but also most certainly haven't forgiven PISTON-1(which they were even warned about by their living god, Metat Aun) and the theoretical doom it would spell for their world someday millenias down the line(assuming it hasn't been indeed disposed of by Metat).

So when ThirdCom Union finally finished a Blink Station in the Boundary Garden system, even though it was meant primarily to service the nominally Union world of Cornucopia notably as it was currently the farthest Union system from Cradle in the most distant line.
And while the Cornucopian were welcoming the Blink Gate..... the Aun who shared a border there with Union instead saw this as a provocation.

So while Union saw the then subsequent destruction of Fansipan station as an unprovoked attack(the asset was purely civilian and technically there were no intent of going to war with the Aun), the Auns saw it as securing their border from what, essentially, COULD pour an entire armada's worth of Union ships at their doorstep IF Union ever was to decide to go on the more forceful path of 'integrating' world again like the SecCom era.

All this to say that there are worlds/civilizations out there that aren't neccessarily gross villains(as there's a lot to be said in the draft books about how reasonable the Aun could be at times, in fact in the eyes of some comparable to Union for how they improved the lot in life of people part of their civilization's core), but still have great enmity toward Union because of history and other factor(even if not meant aggressively, a Blink Station rightfully WAS not an innocent thing to build in Boundary Garden in the eyes of the Aun)

For another example, there could be also precedent with re: the reluctance of the Karrakin to adopt Union culture(for good or bad, these are the Baronies after all) when a notable moment of Karrakin history was when SecCom(after Karrakin radicals had decided to shoot an Union ships during a moment of nationalist fervor that led them to reject Union's influence at the time) decided to go to war against the Karrakin fleet.

And essentially dealt with the larger and more experienced Karrakin fleet in a simple way: by shooting a bunch of kinetic kill rods at one of the Karrakin homeworlds leaving the Karrakin fleet two choices: Break their formations to intercept the rods to save the world but lose their fleet; or maintain their formation to hopefully defeat the SecCom fleet but lose the world.

The Karrakin fleet choose to save their world and were thus defeated(and even then some strikes still reached the planet, destroying some major cities utterly and leaving scars even thousands of years later in 5016u).

And that's how the Karrakin were initially 'integrated' into Union by SecCom so you can imagine the amount of distrust this might still breed in many Houses(even Republican ones) even to this day.

I don’t have anything to contribute but I think this is a fantastic question to ask. I’ll be thinking about it going forward.