Haxorus is outclassed by other dragons in anything it can do, namely Salamence as a Dragon Dance user, Garchomp as a Scarf user, etc. Don't use it, it's bad.
Haxorus' viability is running around C-D tier. There is a huge presence of Fairy types and Poison Jab/Iron Tail aren't solid enough coverage options to deal with them as they won't OHKO 100%. Haxorus' role is hindered due to the state of the tier and how much checks it plus it is unable to perform a role that other Pokemon in the tier already can to a better degree. So it is "viable" but "not as viable as" other Pokemon