Edo Tensei is something that requires the chakra and body and soul of another person. Therefore it's not his power cause he has to use another person. It's not like he can summon them out of dust. And he's using Anko's chakra so he can summon and maintain control of that many edos anyways
Just like basically normal summons are, in fact normal summons are worse, because there is no real compelling mechanism. Any normal summons can in theory just tell the user to **** off and leave.
Yet and I can post the panel (please dont make me do it! too tiresome) it was explicitilly said that summons are part of the power of the summoner, Kishi in his manga wrote that summons are part of the power of the summoner. And these are wilfull summons, the type of summons that can tell you "**** you" and leave, yet they are still called part of the power of the summoner.
Then how much more, summons that are totally controlled by the user and cant leave you and must fight want it or not?
People have a hard time grasping it because of the type of specialization Kabuto has. Different ninjas have different specialities, and usually those that master an speciality will be found at the top 10.
Naruto's speciality is taijutsu, Shisui is genjutsu, Minato space-time, Pre-Juubi Rikudou Sennin fuuinjutsu and Minato space-time.
You never see anyone saying "Naruto without taijutsu is weak" "If Rikudou Sennin hadnt fuuinjutsu Juubi would had won". They are their specialities, in a similar way Kabuto's speciality is the art of summoning.
Summoning is a power branch in Naruto world, like all others and Kabuto just happened to be the most skilled summoner up to date.