Well, when it comes to KNF vs BAF strength I think it's a fairly good assumption that the KNF achieved a greater amount of forces committed against Bretonia because:
1. Kusari only has to deal with internal conflict, and not really with a huge pirate invasion from another side like Bretonia has to (Cambridge)
2. The GMG acts as a further buffer against Outcasts in Honshu
Conclusion: the KNF pulled a lot of forces from New Tokyo and Honshu to achieve numerical superiority in Tau-31. Furthermore, the whole EMP bomb gambit during the beginning of the invasion of Leeds took out a major Bretonian fleet and left the BAF squarely on the defensive.
None of this implies any giant advantages that the KNF enjoyed from the start, nor does it imply a great numerical superiority by the KNF over the BAF (indeed, the war in the Tau systems lasted for about 3 years which can actually be assumed as a long stalemate before the big breakthrough finally happened), but rather that they used their enemy's mistakes against them when the time was right. Which further implies that if Kusari was to get into another major conflict on another front, they'd become just as disadvantaged as Bretonia is now and would quickly lose all their gains. In fact this is exactly what is going to happen in 4.86.
All this is information that can be derived from Igiss's lore write-ups, rumours and ingame news.
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Now this leaves the core Kusari systems slightly less well defended than usual. And then you have Hokkaido, where it is extremely easy to find rumours and infocards that tell you exactly why that system is indefensible, and the reason for that is the same as how the GMG won the 80 Years War against the entire might of the Rheinland Imperial Navy: there are no clear battle lines, the nebula obscures fighters/bombers on all sensors (in the lore anyway, but doesn't apply for player ships unfortunately) save for the short irradiated strip between the two clouds in the middle of the system, and the KNF have no idea where the Dragons are coming from. On top of that, the GMG have helped the Dragons train their forces further in the art of nebula warfare so that they can undermine Samura - and this happened ingame years ago. The tactical advantage is on the side of the Dragons, and seems pretty one sided to me too. So I would rather say that system is the exact opposite of "fairly secure".
I hope I enlightened some people here about the lore.
