Elandal:
The +10% City Attack ability is not the subject of the "marginal" adjective, as you have guessed. Still, I also rather question if the advantage of the bonus on city attack is as large as that of a Swordsman's compared to an Axeman, which some people mistakenly believe it to be.
madscientist:
The Roman Forum's slightly greater number of GPs allows you to bulb or settle a GP earlier, which grants your economy some sort of tangible gain, which translates to a stronger army in whatever way it manifests, hence "stronger" units, albeit in a very indirect way.
In the same way, a Ger also very indirectly boosts your economy, by allowing you to field in stronger cavalry, which you will use to defend yourself more cheaply, or gain new cities.
Don't get me wrong, I like the Ger. I think that it has a greater impact on Mongol strategy and play than the Keshik does. In fact, beelining for Guilds and level 4 (after one battle) Knights is, IMO, a strong play under either Mongol leader. Even against Pikes, a Combat 2 Shock Knight can hold his own, and a Combat 2 Formation Knight is probably better than a Cossack (comes earlier, longer unit shelf life with upgrades, etc, etc.)
But it's not Charismatic. It's not even Aggressive or Protective. It's just a UB, and its only direct application is to a military unit of a specific class. Therefore, given the yardstick of what a "substantial" buff to a military unit could be, it's really just a very slight bump over the Stable. A 2 XP Cavalry bump, to be exact.