At this rate Oda could conquer Korea and China with zero effort
They would be able to win enormous victories over Korean and Chinese armies, but to establish control over the territories would be a fatal mistake. Even in the 21st century with our advanced weaponry and communications, military empires tend to overreach and sputter out after a few centuries.
Expect the following problems:
-Too much of the budget directed towards maintaining a standing army, not enough spent on domestic growth
-To keep the core citizens and the deployed foreign militaries fed and paid, they must extract more territory and resources from the conquered territories. This will result in rebellions and unproductive labor, loss of trading partners, and loss of regional reputation. Blowback.
-The superior military technology will eventually leak, levelling the playing field. This will require arms escalation, which means too much budget directed towards military growth.
-Eventually the ruling government won't have the ability to pay its bills and lose the credibility to guarantee currency. Hyperinflation results.
-Once the homeland takes a downturn, regional governors in the conquered territories will act as independent fiefdoms and the empire collapses.
It's a pattern that has repeated in every historical empire. Macedonia, Rome, The Mongols, The Holy Roman Empire, The Byzantine, The Colonial Europeans, Meiji Japan, the USSR, and is currently happening to the USA.