Jack was the title protagonist of the Samurai Jack narrative, a warrior from the past hellbent on undoing the future that Aku had designed after his disappearance. Throughout the many struggles he had endured, the hero has displayed a number of admirable qualities by which to appreciate him.
However, like any good character, he is not without flaws—and his shortcomings are increasingly evident by the final season. Through juxtaposing Jack's greatest strengths with his most debilitating weaknesses, we can determine how the forces of evil may have possibly defeated him and the ways he was unbeatable all along.
10 Weakness - Vulnerability To Disease
Though it may come as a surprise to many, Jack's immortal body and godlike skills do not grant him immunity to disease. He is as vulnerable to it as anyone else, demonstrated through a memorable encounter with Aku where the mad demon's illness accidentally became his own through ingesting a sliver of his body.
As a consequence, he grew steadily more unwell until the samurai and his sworn enemy were nearly one and the same. Were it not for a group of kind and reverent monks, it would have spelled the end of Jack's journey.
9 Strength - Skilled In All Forms Of Weaponry
Jack may be most frequently seen with his iconic demon-slaying sword, but he is familiar with all forms of weaponry. This is on account of the versatile training he received as a child, acquiring a variety of skills through the old world's greatest masters.
He was even able to adapt to the current timeline's arsenal, demonstrated through his familiarity with guns and battle against the Daughters of Aku at the beginning of season five. Therefore, his updated arsenal allows him to stay relevant with his opponents' increasingly serious threats.
8 Weakness - Foresight
Samurai Jack's abysmal skills in foresight are among his greatest weaknesses and have arguably cost him the most. A profound example comes from when he and a group of scientists were attempting to escape a planet and access a time portal that he could use to return to the past.
Instead of capitalizing on this opportunity, he left his allies to ensure every single one of them would come out of the mission alive. This was in spite of the fact that if he actually used the gateway, Aku's actions—and their subsequent demise thousands of years later—would never come to pass.
7 Strength - Courage
Samurai Jack's courage has steeled him against the galaxy's most terrifying threats. He was perhaps the only mortal capable of defeating Aku even with the sacred blade that had been bestowed upon his father by the gods.
Further, he has never displayed genuine fear in a battle—his only reticence arises from when his comrades might be imperiled. This provides him with the necessary stoicism to win his battles, gradually fighting his way back to the era that he belonged to.
6 Weakness - Hopelessness
An especially prevalent theme in the last season, Samurai Jack's (understandable) hopeless was his greatest debilitation. Haunted by the things he had seen and the duration of his quest, he hallucinated himself begging for his own death.
It was a sobering reminder that regardless of how tenacious one might be, decades of strife and hardship are bound to erode one's mind. Fortunately, he would not give in to despair after meeting someone who he had found a good reason to live for. Ironically, though Ashi was created to destroy Jack, she would save him in more ways than one.
5 Strength - Stamina
Jack's stamina is beyond human, demonstrated on dozens of occasions throughout the series. The most memorable example comes from his earliest encounter with the Scotsman, where the two had battled for the entire day at a dizzying speed, only showing signs of exhaustion the morning after.
Even after their arduous struggle against each other, they still mustered enough strength to defeat the small army of bounty hunters that been deployed to capture them. It illustrates Jack's outstanding endurance, an asset he would need in order to survive the rigors to come.
4 Weakness - Gullibility
Jack's baffling gullibility is a severe detriment and one that has cost him gravely on multiple occasions. The most vivid (and humorous) example arises from when he was consensually swallowed by a "magic" dragon, one which could "return him to the past." After he was inside the belly of the beast, he was crestfallen to learn there wasn't actually any magic.
A second failure is seen through how easily he was deceived by Aku's female disguise. Using it, (s)he managed to kill a time portal guardian, eliminating the hero's safest means of returning home.
3 Strength - Meditation
Samurai Jack's meditative ability is a necessary reprieve from the rigors of his life. It allows him clarity and to banish the many (figurative) demons that ail him.
In addition to cleansing the soul, it also has a more practical purpose. His communion with existence itself was ultimately responsible for returning his sword to him and rejoining the battle to put down Aku for good. Without it, he would not have been able to defeat the ancient evil even if he did manage to get back to the past.
2 Weakness - Ashi
Though Ashi had provided Jack invaluable assistance throughout the last arc, his appreciation and dependence on her directly resulted in his capture. When Aku invoked her demonic nature and turned her into a monster, Jack couldn't bring himself to strike her down. In the short time he had known her, he had grown to love her with all his heart.
It culminated in a one-sided fight, a battle that the Samurai would inevitably forfeit. As a consequence, Aku would have his greatest triumph yet, setting the stage for an execution that he'd insist the entire universe observe.
1 Strength - Temperament
Jack is virtually impossible to provoke, maintaining an even and steady disposition that allows him to think on his feet and avoid unnecessary confrontation. This was seen predominantly through his confrontation with the "Samoorai," a man who had sincerely thought he was just as much of a swordsman as Jack was.
Though the hero would eventually agree to fight him, he would only do under the condition that they used bamboo sticks since he didn't want to kill his opponent (as Jack prefers not to, when it can be helped). Restless to agree to a match however it might manifest, the Samoorai obliged, resulting in a nearly instantaneous defeat.