The combination of Discovery and DirecTV Now doesn't authorize on Roku, Apple TV, or Fire TV. I haven't checked any other devices.
So, it's entirely up to DirecTV Now and Discovery to decide to allow authorization on Roku or any of these devices.
Roku Ultra (4660)
Apple TV (5th gen), TiVo
Roku boxes from every generation.
Apple TV (2nd, 3rd, 4th gen)