Originally introduced to replace Sasuke in his absence, Sai was meant to be an excellent addition to Team 7 in Naruto Shippuden. His skills as an ANBU are second to none, and his seriousness can help lead the team on missions.
However, Sai's lack of emotional and social awareness creates a rift between Naruto and Sakura, followed by frequent fighting. In addition, he fails to adapt to his new environment and interact with new faces. Danzo's training from Root might've been helpful during his time in the ANBU, but it clashed with the Leaf's current policies. Danzo's distrust brainwashes Sai and the other Root members, which only hinders a potential bond between Sai and his new teammates.
9 He Isn't Compatible With His Teammates
Because of his training with Root, Sai isn't used to working in groups. And as a Root member, he isn't used to working with individuals who aren't like-minded. The leader, Danzo, preferred working alone, away from the Hokage's influence, and made sure to instill that teaching into his subordinates.
This independent mindset is a double-edged sword, frequently clashing with the teamwork-based mindset upheld by Leaf Village ninja. Sai doesn't understand the importance of teamwork and sees it as a useless example of emotional attachment.
8 He Has A Hard Time Socializing
Since childhood, Sai has spent all of his time with Root. Danzo's strict and severe training keeps him from traveling on his own, minimizing his social growth and experience.
When he joins Team Kakashi, Sai is unaware of how to function in a team whose value system differs from what he's been taught. He has the skills acquired from Root, but he doesn't know how to communicate effectively with his teammates. Sai can be compared to a sheltered child with no worldly experience, which serves more as a liability than an asset.
7 His Long-Distance Fighting Style Doesn't Work With The Rest Of The Team
In the original Team 7, Naruto and Sasuke would be the ones fighting on the frontlines while Sakura took on a more passive role. Even though Naruto and Sasuke bickered, they exhibited excellent teamwork and could back each other up.
With his background as an ANBU member, Sai, unfortunately, is a long-range fighter and can't help as effectively. His training forces him into the shadows, and he uses the beasts he summons from his art scroll to attack enemies. When he joins Team 7 as Sasuke's replacement, Sai breaks the balance of the team. He does his own thing from afar and Sakura stays back as the medic, while Naruto is often the only one rushing directly into danger. There's not much teamwork happening with this formation.
6 His Beliefs From Root Don't Work With Leaf Shinobi
Danzo's dissatisfaction with the Leaf's diplomatic leadership led to the creation of Root. With this new organization, his less than peaceful methods could flourish and spread amongst the members.
Unlike Sasuke, who lived amongst the villagers, Sai didn't know about the Hokage's leadership because his knowledge stemmed only from Danzo's biased teaching. Sai never questioned Danzo or did further research on the Leaf's history, and this ignorance led him to clash with the Leaf shinobi who supported the Hokage. Leaf shinobi believe in teamwork and protecting their comrades above all else. However, Sai saw it more as a hindrance to the mission's success.
5 He's Been Taught To See Himself As A Tool
Along with casting aside emotional attachment, Sai and other Root members see themselves as disposable tools. They understand that the village's benefit comes before their own lives.
Removing that humanity goes against the Third Hokage's beliefs, also known as the "Will of Fire." The Third Hokage valued everyone in the Leaf Village, including the shinobi who sacrificed themselves. His pacifistic ideals inspired him to protect his citizens during his time as Hokage. Before Sasuke's betrayal, he, too, believed in the Will of Fire, protecting both himself and his teammates. On the other hand, Sai treated himself as a tool rather than as a valued human being as the Third Hokage believed.
4 He's Too Blunt For His Own Good
Sai's opinions are analytical but devoid of any emotion. What may seem like an obvious fact to him will come off as rude and insensitive to others if said aloud.
Like Danzo, Sai believes that Sasuke abandoning the village makes him a traitor undeserving of sympathy. He also points out the futility behind Naruto and Sakura's attempts to bring Sasuke back, emphasizing that it's a "waste of time." Of course, this offends his new comrades because he knows nothing about their previous years with Sasuke and how much he means to them.
3 He Lacks Emotion
Sai learned to suppress his emotions, which made his social skills suffer as well. He only knows about human interactions from books, but he fails to apply what he's learned.
Upon joining Team Kakashi, he unintentionally angers Naruto and Sakura with his lack of social awareness. His lack of social understanding pushed him to insult Sasuke, their former teammate, for being a traitor to the Leaf, which in turn insulted the rest of Team Seven.
2 His Fake Kindness Is Quick To Anger People
Unfortunately, the book Sai reads isn't helping because he takes the information about niceness too seriously. He greets everyone he meets with a smile and polite attitude, although it's painfully artificial.
His book tells him to address someone with a nickname based on the first thing he notices. Of course, the first thing he sees is never anything flattering. He calls Sakura "ugly," and he insults Naruto by questioning his manhood. However, the latter may be intentional.
1 He Uses Unethical Methods In Missions
As a Root member, Sai's comfortable using whatever methods possible to help the Leaf, even if they're not ethical.
His mind is focused on the village's benefit, regardless of how it may ruin his teammates' trust. When Sai seemingly allies with Orochimaru and Kabuto, sworn enemies of the Leaf, he shows no remorse in switching sides. To Naruto, Yamato, and Sakura, it seems as if Sai's betraying the village. However, he's on a mission separate from Team 7's. As ordered by Danzo, Sai infiltrates Orochimaru's hideout to gather intel about his plans.