While Historic Heroes have very high attack stats etc, there is one main difference between HH's and normal heroes that make them so good in battle and that is leadership.
For a detailed overview of normal hero leadership click here
While Historic Heroes with the lowest base leadership (base 30) can almost effectively lead the largest waves possible (375,000 from Atlantis ), very high leadership Historic Heroes can lead massive amounts of troop effectively in defence and in Uprise battles.
An example of how powerful the leadership attribute is would be, lets say if there was exactly the same amount of troops on each side during a very large uprise battle consisting of millions of troops. If one side had a regular hero and one side had a Historic Hero, the side with the Historic hero would absolutely destroy the side with the regular hero because the troops on the side with the regular are not lead effectively while the side with the Historic Hero receive a large leadership bonus.
Historic Hero Leadership
Vasco De Gama
Weak Historic Hero
180 leadership at lv150
((180 x 10) + 150 ) x 180 = 351,000
Medium Historic Hero
240 leadership at lv150
((240 x 10) + 150 ) x 240 = 612,000
Vasco Nu?ez de Balboa
Strong Historic Hero
289 leadership at lv150
((289 x 10) +150 ) x 289 = 878,560
Strongest leadership Possible
Vasco Nu?ez de Balboa - 289 leadership at lv150 + 30 leadership (full Uprise) = 319 leadership
((319 x 10) + 319 ) x 319 = 1,119,371