Well, it'll will have been much easy, that is sure...
1. Because No Edo Tensei which pretty much what's keepin' the Alliance occupied for now.
2. Kabuto said that he upgraded the Zetsuss with Yamato. Even now, the Zetsus suck, so without Kabuto, they will have been detroyed like crushin' tracing paper,.
Now still, Madara could kill perhaps most of the alliance by himself And Sasuke also. perhaps. So that war will not have been more easy but still, I'll say that they will have got much problem without Kabuto...
Because, with Kabuto doing the job, they (Madara and Sasuke) are both resting while they will have surely been forced to enter the battlefield early without the Edo Tensei's help...
It's almost a ladder war for the Shinobi Army
1st step - Tactics (secure yourself and Tailed Beasts and create proper squads)
2nd step - Zetsus (crush them)
3rd step - Need to pass through all Edo Tensei
4th step - Sasuke
FInal step - Tobi...
So if we analyze it correctly, it will been only a 2-step ladder without Kabuto.