Ionian Hetairoi is limited by Independent Action E- (only within the confines of the Reality Marble) and the inability to materialize Noble Phantasms.
Interview:
Nasu: The scary part is, some heroes in this army were stronger than the summoner Rider himself. Although Rider's deeds were great, it was not a testament of his personal martial prowess.
Sanda: So do they have basic Skills and Noble Phantasm as Servants?
Nasu: No, since they were summoned via Rider's skill, it was not necessary to assign them Class based on the rules of the Holy Grail War. However, since there is a limit to Rider's abilities, their Noble Phantasms could not be materialized. If Iskander's army of tens of thousands could pair up with Gilgamesh, who had unlimited Noble Phantasms, the result would be most ideal.
What would be the scenario if thousands of Heroic Spirits armed with top-tier Noble Phantasms (ranked A and higher, with Gilgamesh having Ea at hand) attempt to fight against the top tier above them (Type Earth, Counter Guardians, etc.)