WARNING: The following contains spoilers for Naruto: Shikamaru's Story -- Mourning Clouds by Takashi Yano and Masashi Kishimoto, available in English from Viz Media.
Following the end of the Naruto series, fans were stoked to learn that would be Shikamaru be Naruto's right-hand man as he took up Hokage duty. It felt like a natural fit as they'd been on so many missions together and have always had each other's backs through thick and thin.
In the Boruto series, Shikamaru has done a fairly decent job thus far as Konoha's second-in-command, garnering the respect of his peers and citizens alike. However, in Naruto: Shikamaru's Story -- Mourning Clouds, we get a close look at how unstable he is, learning, in the process, that he might not be as a good a fit for the job as first assumed.
This doesn't take anything away from his accomplishments as a teen, helping end the Fourth Great Ninja War against Kaguya, or disrespect what he's done to help Naruto evolve Konoha; it's just a matter of diplomacy. Shikamaru comes off like a rogue in this novel, which takes place right after the Chunin exams where Boruto used scientific ninja tools to cheat, barreling into the subsequent attack by Momoshiki and Kinshiki.
These issues are dredged up at the Kage Summit, leading to Shikamaru attacking the Mizukage of Kirigakure, Chojuro, and Tsuchikage of Iwagakure, Kurotsuchi, for wanting Konoha's secrets and insulting the council. Naruto's petrified as his comrade almost starts a war, and after it's defused, he chides Shikamaru for this impetuous behavior.
Going after two dangerous leaders and potential warlords like this with his shadow jutsu is very much a mistake. Even worse, this rogue mentality persists. He doesn't respect Naruto enough to stop it or be honest with the Hokage about running secret operations behind Naruto's back -- coming off like Tobirama and Hiruzen Sarutobi, Naruto's successors who plotted against the Uchiha. They lacked an air of transparency and accountability, too, which almost wrecked the Hidden Leaf village. It's surprising to see Shikamaru hasn't learned from this.
Even after the argument, he continues to place spies like Ro in Iwa's territory, not to mention he lets Tento be kidnapped so Boruto could rescue the kid and win them the favor of their feudal lord so he could back them in the Continental Summit. It's outlandish that he'd use a kid as bait, and it's hardly Naruto's style, either. Sure, it's calculated and cerebral, but it's also careless to endanger lives like this.
Of course, as they know each other so well, Naruto discovers everything, eventually reprimanding him for these actions and for underestimating him as a leader. He knows his friend means well but Shikamaru's still a wild card who constantly breaches the trust placed in him. Mourning Clouds also reveals that all this trickles down from his personal life being in a mess as well. Shikamaru neglects Shikadai, smokes and drinks way too much, and the stress from his marriage to Temari also affects him at work. He's easily the problem here.
To add to his problems, the stress is also causing his chakra levels to decrease, weakening his jutsu as well. He ends up realizing that deep down, he's not cutting it professionally or personally. Luckily, by the time the story ends and civil war is averted, he mends fences with both his family and the Hokage. But Naruto knows Shikamaru is capable of making risky moves that could doom them someday, meaning that from this point on, he has no choice but to keep a closer eye on his best friend.