None of the reasons hold up to even a little scrutiny.
Among its latest reasons for attempting to ban transgender people from the military (again), the Trump administration points to potential disruptions to something called "unit cohesion" — basically, how well members of a troop work together.
Secretary of Defense James Mattis wrote that allowing trans people to serve in the military "could undermine readiness, disrupt unit cohesion, and impose an unreasonable burden on the military that is not conducive to military effectiveness and lethality."
Using that memo, Trump announced that trans people would be "disqualified from military service except under certain limited circumstances." The entire process was clearly just a way to reverse-engineer a rationale for implementing his impulsive July 2017 tweets on the subject.