I traditionally watch Netflix on my AppleTV Gen 3, the quality is always excellent, It is connected wifi but is near router. When I got my XBox One, I connected it wifi at the same tv as the AppleTV. I had good video on day 1, but low quality on day 2, but also with Amazon Prime video. I moved the XBox One to ethernet and the video got better for a period. Unfortunately the quality is often low.
AppleTV 5800 at 1920x1080 (Went immediately to that, dropped to 3800 shortly and back up)
X1 (first test) 3800 at 1920x1080 (started at 5xx and then climbed)
X1 (second test) 5800 at 1920x1080 (started at 5xx and climbed)
I would guess that Netflix may be treating the client differently - but since I also have problems with Amazon Prime video, I would blame the X1.