An Infiltrator Army would also have Archers, Cavalry and Infantry. While they might not be as strong as an Army composed out of more battle-focused classes, their strength should come out of the disruption and chaos they can cause next to it (Stealing Tax Gold for extra income, stealing/burning food, lowering militia, destroying fortifications, stabbing Nobles, delaying travel). So an Infiltrator Army probably should not focus on attacking the enemy head-on in battles, but seek their advantage through such options and slowly whittle down the enemy while avoiding the big blobs attacking each other, at least not until their actions take effect.
But yes, lifting bans etc. would be a major diplomatic point and challenge for such a Realm. And lifting bans should probably be on the top of the list for any negotiations of any kind between realms (Prisoner Agreements etc.)
