Comparing OT and NT severity of divine judgment is a category error: the OT covers thousands of years of history while the NT covers only a brief first-century window

The OT covers a massive amount of history, so we see both extreme judgments and long periods of delayed judgment (e.g., Amalekites, Moabites, Canaanites). The NT covers only the brief first-century window and does not tell us what God was doing elsewhere in the world. The comparison between OT and NT judgment frequency is therefore not a fair one.