I believe it would be more nearly correct to say that the Sui rulers demanded too much of their subjects, not so much in military conquest, but by forced labor and excessive taxation.
The Sui rulers were responsible for the construction of the Grand Canal, which is still extant, and runs for 1240 miles. This canal allowed trade routes to travel by water north and south. Chinese rivers tend to run east and west, so water transport was not feasible before construction of the canal. Functional as the canal was, it was extremely expensive. Over two million workers were required to work on the canal against their will. This, together with high taxes on peasants caused a great deal of discontent. There were some military defeats, particularly in Korea, but these only exacerbated an already bad situation.
There were a number of rebellions, primarily by peasants, still the Sui dynasty did not end by military defeat. The last Sui Emperor, Sui Yangdi, was murdered by one of his ministers. At that point, a rebel leader, Tang Taizong (who had murdered his father and brothers to clear the way for his rule) proclaimed himself Emperor and founded the Tang dynasty.
So it was not military reversals so much as overworked peasants and overextention of taxes that brought down the Sui dynasty.
As is so often the case with rulers of empires, the Sui dynasty tried to conquer too much territory and to do too much in the way of massive public works.
The Sui dynasty tried to fully restore the empire and its prestige. They attacked to the north and defeated the Chen dynasty. They attacked what is now Vietnam. Worst for them, they attacked what is now Korea, creating a huge and expensive army that sapped the dynasty's resources.
The dynasty also outspent its resources on public works. It rebuilt the Great Wall of China. It built huge palaces. It dug long canals. All of this took a lot of money and man power.
The excesses of the Sui Dynasty, then, consisted of excessive spending. Paying for all of this put too much pressure (through taxation) on the people and weakened the empire.