Problem would be the major tweakage to explain how the Fire Emblem "Servants" would get buffed in F/GO or how the Fate "Heroes" get nerfed in FEH.
P.S. Marth clones and Saber clones only.
There is no explanation needed seeing as there are so many servants who never ever had super powers in any legend now it's not even funny. Basically it's pretty damn obvious at this point that just getting summoned as a servant gives you super powers.