I have no particular opinion on Porygon-Z. I tend to feel like it's an example of a fun STABmons 'mon -it gets a lot of benefit from the meta, but with Shell Smash gone it's just powerful, but whether it's broken I dunno.
Kyurem-Black really does need to go. Between getting an actual Physical Ice STAB (Often enough to push it over in other metas) and gaining Dragon Dance, all while Teravolt means it laughs at your Unaware walls, the Dragon Dance+Roost variants can become basically untenable to deal with outside of being something like a full fakespeed team. It's just too bulky to easily stop it from getting started, and with Substitute it can ignore any attempt to drop Toxic on it or hit it with a Burn, especially since it resists Scald/Steam Eruption and so its Substitutes will often be able to shrug off those attacks.
My current team won't be much affected either way because it is a full fakespeed team, but that's no argument to keep Kyurem-Black in.
Kyurem-Black really does need to go. Between getting an actual Physical Ice STAB (Often enough to push it over in other metas) and gaining Dragon Dance, all while Teravolt means it laughs at your Unaware walls, the Dragon Dance+Roost variants can become basically untenable to deal with outside of being something like a full fakespeed team. It's just too bulky to easily stop it from getting started, and with Substitute it can ignore any attempt to drop Toxic on it or hit it with a Burn, especially since it resists Scald/Steam Eruption and so its Substitutes will often be able to shrug off those attacks.
My current team won't be much affected either way because it is a full fakespeed team, but that's no argument to keep Kyurem-Black in.