Nasty Plot Hydreigon is decent however it has a lot of issues in the metagame right now that make it overall less effective and overall less viable than its Bulky Defogger set. So I want to go over 1 large flaw of Nasty Plot Hydreigon in the metagame to add to Katy's already great answer that make it a less effective wallbreaker!
The first and biggest flaw of it, is that it is more easily pressured and forced out. New additions like Garchomp, Pheromosa, Latios, Tapu Koko, Tapu Fini and Kartana, and even previous checks like Clefable and Blissey have made its job significantly harder due all of them being pretty common, and good Pokemon in the current metagame, which gives it significantly less and made its job of breaking much harder.
The second is the fact that despite how powerful and threatening it is when it does get the NP off, the lack of immediate powerful before hand really hinders it in its ability to hit hard. Outside of a few Pokemon such as Slowbro and Spectrier, Hydreigon doesn't really force many things out like it used to considering that this metagame is significantly more offensive combined with its lack of setup opportunities now, while its devastating when it does get the opportunity to wreak havoc, the opportunities are simply not there unfortunately.
Hope this helped with knowing NP Hydreigon's viability!