Hi. UU Crash Course.
In M-Houndoom's case, it benefited from the legalization of Drought in UU. In past iterations of UU, Drought was banned, so M-Houndoom's NP set was manageable. However, in Gen VII, M-Houndoom's new way to abuse Drought with Solar Power allowed its NP set to plow through most of the metagame with Fire Blast, including Blissey. It became a broken aspect of the metagame and was locked away (for good reason, too).
Porygon-Z's main issue was that the Z-Conversion set made it really hard to revenge kill for multiple reasons. First, you get the +1 across all stats, which makes it much harder to Revenge Kill with neutral moves. Second, Conversion's mechanic of switching Porygon-Z's typing makes it really hard to Revenge kill as well. Variants either ran Thunderbolt or Shadow Ball in the first slot. After the Z-Conversion, its typing changes to either Electric or Ghost. When one plays against it, there's no way to concretely determine the best way to Revenge kill it. I'm pretty sure there were a plethora of other reasons, but I personally think these two are the most important ones.
Staraptor is just incredibly problematic because of CB Brave Bird and Close Combat with Reckless. With the damage boost from Reckless, very few Pokemon can actually check it reliably, especially since it sits at a considerably high speed tier.
I can't really speak on the other three, but for Diggersby, I think its SD set invalidates Bulky Offense and Balance builds with a combination of Return/Earthquake/Quick Attack.
Hope this was helpful.