Warning: SPOILERS for Star Trek: Discovery season 3, episode 3, "People of Earth".
After one year apart from them, Commander Michael Burnham (Sonequa Martin-Green) rejoined her starship's crew on Star Trek: Discovery, but this is a mistake that could prove to be detrimental in the long-term. When Burnham and the Discovery traveled 930 years into the future, she was separated from the ship and arrived in the year 3188. Discovery didn't emerge until a year later but during that time, Michael had admittedly moved on and forged a new life for herself.
Though she has heroically saved all life in the galaxy, Michael Burnham's Starfleet career is checkered, to say the least; her adoptive father Ambassador Sarek (James Frain) placed her on the U.S.S. Shenzhou under Captain Philippa Georgiou (Michelle Yeoh) when Michael was rejected by the Vulcan Expeditionary Group because she is human. Commander Burnham distinguished herself and became Georgiou's First Officer but her tragic error in judgment instigated the Klingon War of 2256-2257. Michael was imprisoned as Starfleet's first mutineer yet the Mirror Universe doppelganger posing as Captain Gabriel Lorca (Jason Isaacs) transferred her to the U.S.S. Discovery as a science specialist. Burnham was redeemed by spearheading the end of the Klingon War and she regained her rank of Commander. In Star Trek: Discovery season 2, Burnham and Commander Saru (Doug Jones) were in contention to be Captain of the Discovery - an issue they settled when Michael wisely rescinded her candidacy, allowing Saru to finally get promoted to Captain.
But unlike Discovery's newly-arrived crew, Michael lived one full year in the 32nd century and the experience changed her in profound ways. Unbound from her duties as a Starfleet Officer, Burnham became a courier and traveled the galaxy, searching for her lost ship and for answers about the Burn that led to the collapse of the United Federation of Planets. In addition, Michael found romance with her "true friend", Cleveland "Book" Booker (David Ajala). As Ensign Sylvia Tilly (Mary Wiseman) perceptively noted, Michael is different; she was "lighter". Emperor Philippa Georgiou also saw the new shades to Burnham and she correctly surmised that her surrogate daughter enjoyed the freedom beyond the duties she had always clung to for purpose and identity. Michael admittedly "let go" of Discovery and had to face the possibility that she would never see them again. But now, she's reluctantly back aboard her starship, and though Michael wisely declined a promotion to Captain, signing on as Saru's First Officer was the wrong move.
By accepting the position as Discovery's XO, Michael is telling herself it's the best way to pursue her mission to find and reunite the various isolated pockets of the Federation - and she's right. But by rejoining Discovery, Burnham is attempting to fit back into her old life and pausing the new, more liberating, and personally fulfilling one she built alongside Book. Michael sincerely admits that she's not the same person she was when she left 2258 and her hesitancy about returning to Starfleet is evident. Yet, out of loyalty to Saru, Burnham chose to become his First Officer, which could be a decision Michael and the Discovery could come to regret.
Now, it would be one thing if Michael stayed aboard the Discovery in a different capacity, such as the specialist she was in Star Trek: Discovery season 1. But Michael serving as First Officer means taking responsibility for a crew she doesn't feel quite the same attachment to, and enforcing Starfleet regulations she no longer feels necessarily bound by - as evidenced by how Burnham and Book had no qualms about going behind Saru's back to capture the leader of the dilithium raiders in Star Trek: Discovery season 3, episode 3, "People of Earth". Of course, Michael is the main character of Star Trek: Discovery so she has to be part of the starship's voyages, but accepting the role of Saru's Number One is ill-advised - and they both know it deep down.
Worse, Michael's presence could have a snowballing deleterious effect on the crew, who are obviously in mourning and traumatized by jumping 930 years to a darker future than they expected. A First Officer like Burnham who has personal doubts about being in Starfleet sets a poor example, especially when crewmembers like Lt. Keyla Detmer (Emily Coutts) are already being openly insubordinate. In the Short Trek "Calypso", the U.S.S. Discovery was long abandoned by its crew and Michael as XO could be the seed that ultimately leads to the Starfleet Officers one day leaving their ship behind. At the end of the day, Captain Saru needs a First Officer he can completely trust and depend on. But despite his love and respect for Michael, that just isn't her at this point in time and this could lead to a future disaster in Star Trek: Discovery season 3.