It's of course bad writing that the Nightborne are joining the horde. They have recently been through a revolution and a demonic invasion. And the first thing they do after is joining the Horde AND making themselves enemies of the Alliance. I cringed listening to Thalyssra say "For the horde!" during the recruitment quest in 7.3.5 it's as bad as blood elves joining the Horde years ago. But for gameplay reasons I accept it.