They're not mentally ill (gender dysphoria is a mental illness as defined by the DSM IV, the last DSM that actually was worth anything, despite psychology being a meme -- but this isn't about that), they're on hormones that make them emotionally unstable (estrogen is necessary but is HUGELY disruptive and causes major depression, and the doses of testosterone that FTM transitioners are taking is enough to cause aggression, mood swings, etc.). If not managed the drugs HUGELY fuck up unit morale and cause division among the enlisted men... but all people who are severely injured and require constant maintenance to keep them from getting infected and dying will suffer... and many more reasons for why we should be more sensitive to soldiers who transition that I cannot be bothered to explain.