Arrogance has long been the Achilles' Heel of heroes and villains alike during the history of Dragon Ball. Frieza, Gohan, Majin Buu, Vegeta and even Goku -- if the most powerful characters could reign in their egos, the battles would be much shorter. However, Vegito, the Fusion of Goku and Vegeta, has proven that he does not share this weakness, despite being the most arrogant of them all. His return in Dragon Ball Super proved that Vegito's only true weakness is the time limit of the Potara Fusion.
When Vegito made his debut in the Buu Saga of Dragon Ball Z, he spent the better part of four episodes almost literally turning Majin Buu inside out. Vegito's borderline sadistic glee in pulling Majin Buu apart like a pink banana peel initially reeked of classic DBZ arrogance. Vegito's cocky attitude did get him briefly turned into candy, but this did nothing other than giving him new ways to torture his helpless opponent. Even the Kai watching the battle assumed Vegito's ego had gotten the better of him.
Unlike Vegeta and Gohan's separate instances of refusing to destroy Cell when they could have, Vegito's game of "pin the tail on Majin Buu" wasn't the result of his power going to his head. The Fused warrior was simply biding his time, trying to goad Majin Buu into absorbing him so he could rescue his sons and Piccolo from inside Buu's body. Although had Vegito known that his Fusion had a time limit, he might have spent less time folding Majin Buu like a cheap lawn chair. He might have even just destroyed him outright and banked on the Dragon Balls being able to revive everyone, even if they died as a part of Buu.
Vegito's next appearance, this time in Dragon Ball Super, reaffirmed that overconfidence is not his weakness, rather his time limit is. The Fused Saiyan didn't dawdle in his battle with Merged Zamasu, as the immortal god was much closer to him in power than Buu. Vegito was still the superior fighter, but he recognized he had significantly less room for showboating, restraining himself to some minor trash talking. Unlike with Buu, Vegito quickly went for the kill with his Final Kamehameha, and when that didn't work, he switched to another finishing move at the same moment that he separated back into Goku and Vegeta.
Once again, Vegito was unable to defeat his opponent before his time ran out. Even though he managed to last the full hour against Buu, it wasn't enough time to complete his rescue operation. Against Merged Zamasu, he only managed to stay Fused for ten minutes, as Goku and Vegeta had become exponentially stronger since the last time they Fused. The Potara Earrings have a critical weakness in that they are almost uselessly unstable when used by beings of incredible power.
The power of the gods has been the predominant theme in Dragon Ball Super, with Goku and Vegeta each capable of wielding god ki. Even the gods themselves are impressed by the ability of these two mortals to command such tremendous power. However, Goku's and Vegeta's combined god power is too much for a single mortal body to endure, resulting in Vegito's premature separation. The manga speculated that Vegito separated because Goku was stronger than Vegeta, resulting in an unbalanced Fusion, but the end result is the same: Vegito's time limit is his biggest weakness.
If Vegito's Fusion was permanent like Goku and Vegeta were told that it was, he could have easily rescued his friends and family from inside Buu, and Kid Buu would have been no match for him. Merged Zamasu's immortality meant that there wasn't really anything Vegito could do to stop him, even though Vegito was clearly stronger. Vegito's time limit is a necessary nerfing of what would otherwise be an invincible character. It is a testament to Vegito's unbelievable power that Akira Toriyama himself has to step in and pull him off his opponents.
