While in Hegel, alienation and unification can be seen as opposites (the alienation ultimatly disappears only in the final unification), even for Marx this is not true: the capitalist mode of production unifies (almost) everything, but alienation remains: the products of the worker are not her own, but present themselves as an alien and potentially hostile power. In Marx, however, the unification is not total, because the remaning alienation (although the later Marx no longer uses that notion) is understood to be visible and the necesessity obvious to abolish dispose of the mode of production that creates said alienation. This is where some 20th century Marxists disagreed: It seemed that the unification indeed tended to become total, and the question became how to explain that.
Unification means that everything fits together, everything can be explained by something else and, therefore, everything that is can be justified. If alienation can be justified, any hope is lost. Debord’s project hinges on the hinges on the possibility that the unification is a wrong one.