Well, yes, some mono users build into all teams, but this is a process that takes a long time to do. Usually they start out with 1 team and max the grid, or nearly max it and then add on the next team after.
By the time a mono user finishes all of their teams, a rainbow user would have nearly a full mono grid for all of their teams as well. Some rainbow players might even check future events for the best Assault SRs and add those to their grids too.
Endgame they would essentially be the same, the only difference is the rainbow user went through a lot less stress and resources to get there.