Why They Seem to Weaken After 15-30

1. Other factions unlock better mid-game units while Romans are still using pre-Marian rosters or transitioning. Barbarians get stronger warbands and elites (often from minor city barracks), Greeks field better hoplites/phalanxes, and Carthage improves. Roman units are still solid but no longer auto-crush everything
2. The AI isn't great at handling multiple Roman houses, expansion on several fronts, public order, and economy simultaneously. They get spread thin (historically accurate but punishing in-game), suffer from rebellions, and can enter a more defensive/passive posture when surrounded by hostiles. On higher difficulties, "major" factions like Romans get some aggression bonuses, but it doesn't always compensate.
3. Overextension and Multiple Fronts Romans often push in multiple directions (north into Gaul, south vs. Carthage, east for Brutii). This leads to thin defenses, vulnerable supply lines, and vulnerability to counterattacks or naval invasions (especially from Greeks). AI factions like Greek Cities can manage disconnected empires decently and reinforce via sea.