#4 Tactical imbalance
This is perhaps the most important aspect when a team is concerned. A team can have all the best players in the world, but it amounts to nothing if there is no tactical balance in the system. The Real Madrid side post-Vicente del Bosque is a testament to the aforementioned statement.
That Real Madrid side had all the best players in the world, but were a sheer disappointment when it came to winning trophies. As the saying goes, too many cooks spoil the broth—and with Cristiano Ronaldo and Lionel Messi in the same team, that could just have been the case.
Both these players are never—or rarely—given any defensive duties by their respective managers. It is simply a waste of their attacking abilities if they have to track back to defend. In such a scenario, playing the two in the same line-up could mean that the system would have two players who do not defend.
Or, each of the two could have to share their portion of doing the dirty work. In any case, it could have invoked a disastrous consequence that could have resulted in the team failing terribly.