So many things.
12 people "sieging" the City of Venice. 200 people "sieging" the City of London. It's so retarded you can't even make a case for it, and it didn't happen. These aren't Conquistadors with guns and Smallpox. They raided undefended monasteries until West Francia changed defenses and Wessex implemented massive standing armies and brutal fort system defenses. They got their teeth kicked in a few times and realized the tactic had stopped being profitable enough to balance against their losses, wherein they changed tactics (because they weren't morons).
When they added Viking raids, they failed to modify the game to reflect the counter-mechanisms that nullified them. Wessex had 1/4th of its population under arms, because they had been hit by the vikings so much (not just raids) the citizens and nobility would pay for it. Standing army, not a personal retinue, no time spent "organizing the troops".
Also, "the game mechanics are actually reflective of the real-world"? The Viking nations are losing a Grande Armee every year. They get absolutely butchered every year over and over for centuries. When the real Grande Armee died it ended that country's status as a World Superpower (forever?), but apparently these Go-Getters are using Thor's Cloning Vats to replicate. Spare me the "real world" bs as a basis for the arguments you are making, and possibly look up Alfred's military reforms. A Nation-State that gets hit by foreigners intent on plunder or terror responds aggressively, but CK2's "real-world" serves as a model of waiting around century after century for the next raid to land.