A list of biases toward oligarchy:
Plato will always see more problems with the other societies since he most highly valued The Republic.
Plato believed that Oligarchy had more problems than Timarchy. However, he is biased because he was more favorable of Timarchy because they more closely mirrored Sparta, and Plato loved Sparta. He admired Sparta for their aggressiveness and warlike tactics, two things that Oligarchy generally lacked.
Plato believed that the distribution of political power prevented the virtuous poor philosophers from influencing public life. Since Plato was a philosopher, he described the fact that poor people had a difficult time joining the oligarchy as a negative thing.
Plato, being a poor man, believed it was the State’s responsibility to decrease the wage gap. He believed that oligarchs were selfish and rash and did not take into account the underclass as they were too focused on getting money for themselves. Plato thought that the very nature of an oligarch seeks to create inequality and that their ruling techniques would lead to an oversized underclass that is destined to rebel.
Plato was a huge fan of the Spartan military. He believed that military campaign was a sign of power and strength. However, this caused him to view the oligarch’s lack of military power as a negative thing because oligarchs were unwilling to give weapons to the majority (the poor) because oligarchies fear revolution.