If you think JK Rowling is an awful transphobe there is zero cost in saying so - apart from having to justify your view (and it would be the right thing for people with integrity to do)
It would be powerful and important (but it comes at some personal cost)
In the longer term it costs the integrity of the whole human rights project, as you look at each other and at your leaders and know that this is a movement of people who don't speak up for what is right when it counts