Transgenders were only allowed to serve openly a year ago. I have no idea what effect this has had on general military preparedness. I'm assuming the people that would be the best judge of this are people in the military, and I would defer to them. I'd like to assume Trump is doing the same, but it would certainly fit with his personality to ignore them. I just don't know. There may have been zero problem associated with this, or they may have been numerous ones that may not have been brought up because of fear of crossing the Obama administration on one of their planing policy pushes.
I agree that prior to the ban being lifted in 2016, there were still transgendered people serving while in the closet, so to speak, and I agree it would be wrong for their status to be affected by this new decision now that they're open about their status. This is just another example, of which there are many, of the problems with a Presidential administration rapidly changing or undoing policies without considering the consequences or giving adequate notification. It certainly isn't unique to Trump but he seems to have a flair for it. Major (or minor, for that matter) policy decisions shouldn't be made by Twitter.