If I recall, the Marches pulled in and stayed strong for the duration of the Rebelliion and Virus, then actually allied with the Aslan. Not a force the Swords could threaten. They were better and smarter to just be glad that they had the Marches to protect them from Virus.
As for their enemies being martially incomeptant I see where you get the idea and agree to a point. Largely though I see it, in the Spinward case, of no foe serious about conquest.
First there are the Zhondani. A mighty force by any measure and imo quite capable of handing the Imperium its arse anytime they wanted to. That is the key though. They never wanted to. They were quite content to make certain the Imperium had no desire to grow, and in that they were successful. What's more they are much more interested in the core. The true core, not the center of the Imperium.
Next in that area are the Vargr. They too have no real desire to take on the Imperium, they have all the room they need to grow the other way, besides there's better things than empire building in their genes. I for one never really liked the hokey religion angle they have.
Then the other way are the Aslan. They are a potential problem, and in the Rebellion it was one that grew rather rapidly iirc. The buffer of the rift there helped slow that confrontation perhaps, one that in another era would have seen a very bloody war, one that would have devasted the Marches regardless of who survived. And that's what it would have come to, survival, not victory. All imo of course.
And of course there are the "minor" players in the region. The Darrians, The Swords, and a couple other even smaller polities. The fact they are minor means they are no threat and can't mount a threat on their own. They will gladly back the side that promises to win and make them stronger but they won't be starting anything with the big boys, short of perfecting the Maghiz or something similar or course.
As far as the Imperium always winning, well, you need to study your history a little more
They have as many or more losses than wins.
There were the (first) Rim Wars they eventually lost to the upstart Solomani ushering in The Rule of Man. And that being the much larger (by several magnitudes) Imperium.
Over in the Marches the Imperium lost ground in the 1st, 2nd, and 3rd Frontier Wars, pretty much did nothing in the abortive 4th Frontier War, and faired little better in the 5th Frontier War, gaining a few worlds along the Sword Worlds border.
All in all I'd say the mighty Imperium looks pretty bad at fighting wars, no doubt due to it's size.