I have to say I disagree not entirely, but a majority. The russian revolution took a few years sure but thats a country.
And look how long it kept. The USSR was on the brink of collapse in the earliest days of WW2. And look at how the Reich failed at ensuring that collapse to happen. Even after renewed growth and explosive advancement, it lasted for about 40 years. Look at Russia now. Every republic that makes up the Russian Federation sees bloodshed and insurgency. There are daily terrorists attacks, and the country almost completely collapsed in the 1990's.
But a single example of how fast conquest can be on a country scale doesn't negate the almost impossibility of planetwide conflict yielding victory for one side. No more than me listing the several contradictions such as the Hundred Years War, the on-again-off-again Crusades, the on-again-off-again wars for the Americas and Orient. The continent of Europe engulfed in constant bitter struggle since the collapse of the Roman Empire- which still is a hair away from major conflict (look at the Balkan Powder Keg and the worsening situations between Turkey and Greece).
Look at the very planet itself. It's been thousands of years, and there are over 300 countries of which at any one time half of them are involved in some kind of conflict.
What do we really know about the planets we colonize/take over. Are they farming planets, industrial, military. Whats there population I dont think it would take a billion soldiers to take over a planet with around 1,000,000 civilians(and others).
Yet the UN can barely root out 10,000 insurgents in Afghanistan, even with a vast superiority in both technology, numbers, cultural significance, and economic relevance. Poppy farmers are scaring away a superpower that landed on the moon.
Those farmers on that planet you're trying to conquer will also now have access to whatever technology you bring to fight against them, and that's not even counting the TEC or Vasari stepping in just to spite you.
And the technological gap between the three factions is not wide enough to render a shock trooper deployment successful just on those standards. Heck, it might in fact be worse for the invading army to win quickly, as now it has to fight insurgency and not an identifiable foe on identifiable fronts.
That's not considering a cold hard truth that the Sins universe is locked in the current type of fighting out of necessity- that fighting being almost purely naval. Again, raw materials and numbers are so spread out towards the already established doctrines that diverting even a single percent away from that would drastically weaken whatever faction tried, and that single percent would not make enough of a difference in invading a portion of a planet, much less the whole system.
Sins was always about imagination, and I think even if it doesent describe HOW we take over the planet, if we were to say have shock troopers it would up to us generally how that planet was seized. See what im saying, if culture can lead to revolution of a planet I think taking over military complex's and annexing the local government coulden't be much harder.
Just imagine what kind of land-based warfare products could come out of the advents weaponry(sorry for the bias towards the advent im just a space commie). Culture tanks!
The weapons don't make much of a difference for the Advent enough to break a galaxy-wide stalemate, what makes you think conventional means will make it easier for them to conquer a planet? Even the TEC has an easy enough time fighting them off through tactics and numbers. On a planet they don't have a vast universe of space to hide across, especially if they're occupying forces. They just painted themselves the biggest target in the universe, and should the TEC or Vasari break their fleet in orbit, you can kiss that invasion force goodbye.
And the very relevance of cultural insurrection and unhappy workers rebelling is one of the exact obstacles that would need to be hurdled for ground invasion. Especially if the planet your troops are invading are happy with their current faction. Eventually they will turn on you and reverse engineer your carefully plotted invasion, and now your faction is vulnerable to the opposing forces.
Remember that in the Sins universe, planetary conquest is purely about gaining resources and taking resources away from your adversaries. Even for the Advent, focusing on a war for hearts and minds is considered old-school warfare, as the rest of the galaxy is locked into bitter struggle of less humane proportions.