Rotom-H - Volt switches out on it
Mega-Manectric- Only a check/counter if it remains in regular form (which is complete ass) otherwise
252+ SpA Choice Specs Magnezone Volt Switch vs. 0 HP / 4 SpD Mega Manectric: 113-133 (40.2 - 47.3%) -- 50% chance to 2HKO after Stealth Rock
Swampert- I'm pretty sure all Magnezone's carry HP grass but maybe you're a prediction god idk
Quagsire- See Swampert
Gastro- No point running HP ice on Salamence hits dragons for the same damage as HP ice (besides Zygarde which still can't switch safely into a Flash Cannon)
Nidoking/Queen-
252+ SpA Choice Specs Analytic Magnezone Flash Cannon vs. 4 HP / 0 SpD Nidoking: 355-418 (116.7 - 137.5%) -- guaranteed OHKO
252+ SpA Choice Specs Analytic Magnezone Flash Cannon vs. 126 HP / 4 SpD Nidoqueen: 319-376 (90.6 - 106.8%) -- 81.3% chance to OHKO after Stealth Rock
I'd hate to see an unsafe switch in.
Celebi- Easy volt switch into appropriate check/counter
What you seem to assume is that because Magnezone doesn't beat a mon 1v1 if you are both in at the same time e.g. coming in on a double down, that it is getting countered. The whole point of Magnezone is to come in on the useful resists it has or on walls that can't really damage it and to hit back the other team with extremely powerful analytic specs attacks (as analytic boosts the power of moves when the opponent switches). However, there are Pokemon that can come in on Magnezone such as Celebi, Porygon2, Umbreon. But what happens when the opponent volt switches into say, Mienshao. Then all of a sudden, your 'counter' to Magnezone has turned you into a sitting duck to Mienshao. This is why Magnezone was broken, not only could it kill things but it could also leave you in an extremely advantageous position against so called 'counters' to Magnezone. With the few mons that did counter it, a simple volt switch could cause almost force another mon to die without Magnezone even having to kill it.
http://replay.pokemonshowdown.com/uubeta-94224182
Here is an example of a game my friend was playing with a team I made. The idea of the team was to exploit the few switch ins Magnezone had and punish said switch ins. This type of strategy is known to most as 'Volturn'. Here the opponent had two pretty solid answers to Magnezone in Celebi and Chansey, however, what he didn't have a solid answer for, was the imminent Volt Switch. From this, you can see just how easily Magnezone was able to bait him into mons that Mienshao could destroy and at one stage, he was in a cycle of around 5-6 turns with Magnezone and Mienshao gaining free momentum for each other.
So yeah I hope that kind of explained a bit better why Magnezone was considered broken by the UU Council :]
Edit: And if you say 'oh but if he had a Ground Type'
http://replay.pokemonshowdown.com/uubeta-94229585
Believe me, the Ground Type didn't help :]
I'd also like to add that it was a pretty important member to Drag-Mag because as you can see with the ban list a lot of them were dragon types which Magnezone often assisted in clearing the field of mons they generally come into trouble with as he can easily be tailor fit to deal with.