I have to agree with all the people that said UU:
Magnezone is really strong first of all, there is no denying in that. With base 135 special attack, Choice Specs and even a possible Analytic boost, nothing can really safely switch into it. It's typing is also great, while it gives some unfortunate weaknesses which I will go over later. It's steel electric typing gives it a slew of good resistances like Flying, Psychic and one of the most important things, it resists the famous Boltbeam coverage.
While all of those things sound great it isn't without it's fair share of flaws. While it is a difficult Pokemon to straight up counter, checking it is a whole lot easier with Magnezone's low speed. You can run a Choice Scarf to get rid of it's mediocre speed but then you are being counterproductive with Analytic ( switching opponents can still be punished with Analytic however. Also nubs don't use Magnet Pull since there are barely any Steel types to trap ). As I mentioned checking Magnezone is fairly easy as with it's common Fire, Fighting and Ground weaknesses like Kokoloko mentioned. Checks to Magnezone include Chandelure, Arcanine, Entei, Darmanitan, Flygon, Nidoking, Mienshou and any other Fire, Fighting or ground type you can think off. Also like Kokoloko said it wears down over time with repeated switch ins into Stealth Rock, resisted attacks that wear it down over time and possibly Spikes damage. Add to that the fact that Magnezone doesn't have reliable recovery outside of Rest and it probably won't last the whole match.
Now even with all the negative things I said Magnezone is still a great Pokemon but it isn't overpowered so Magnezone for UU.