How does the description of Israel's oppression suggest it was more severe than those oppressions that preceded it?

In the Othniel cycle, the enemy Cushan-rishathaim from distant Aram-naharaim never actually appears as the subject of a verb; the narrator simply says that the Lord sold Israel into the king’s hands and that the people of Israel served the king (Judges 3:8). In the Ehud narrative, the narrator not only attributes the oppression to the Lord and notes that Israel served Eglon the king of Moab, but he also ascribes hostile actions to Eglon. Yet he stops short of actually using a verb whose meaning is oppress (Judges 3:12–14). Now, however, the narrative explicitly states that Sisera oppressed…cruelly the people of Israel for twenty years. The verb used also shows up in the exodus narrative (Exodus 3:9), which could suggest that Sisera’s treatment of Israel was of a similar kind as Pharaoh’s. The addition of the adverb cruelly expresses the severity of the oppression against Israel. Finally, the reference to nine hundred iron chariots conjures up a rather terrifying mental image and suggests the apparent invincibility of the oppressor.¹