What bad things did the unification of the country bring to the people of China?

If we take the first unification of China into a single state in the 3rd century BC, then the disadvantages were as follows:

The beginning of the great construction projects, which attracted a huge number of people, in fact it was oppression, it intensified. The tax burden has probably also increased, as construction projects required an increase in government spending.
The cost of building luxury palaces (like Epan).
Persecution of the scribes, 460 buried alive, such a persecution of dissidents.