In a nutshell, it can run any set it wants and even in the face of ten counters, it can still succeed. From specs to wp sg cm under screens. Aside from that, another thing that truly makes Magearna a pain in the ass is its bottomless movepool. It has access to thunderbolt, ice beam, energy ball, focus blast and flash cannon to name a few which is further compounded by its ability that basically functions as a special moxie. If you want the full details, you can find it here
Thanks to the Steel/Fairy typing and 80 / 115 / 115 bulk, getting in safely is not a problem for Magearna either. While we recognize that Pokemon like Assault Vest Slowking-Galar and and bulky Volcarona are able to minimize the effectiveness of Choice Specs Magearna, they still allow for free Volt Switches into more threatening teammates. This makes Choice Specs Magearna a suffocating presence as even in match-ups where it is not necessarily going to be the breaking force in gameplans, it is still seen as a major catalyst
Choice Specs Magearna is arguably the most consistent variant of Magearna. 130 base special attack makes it one of the strongest special attackers in OU, which goes a long way when paired with a strong STAB move like Fleur Cannon and a slew of complimentary moves. In terms of the other attacking options, Flash Cannon is oftentimes utilized as another STAB option. In addition, Focus Blast is a strong coverage option used to devastate Steel types like Heatran, Excadrill, and Ferrothorn. Finally, Volt Switch is a staple on this set to generate momentum and threaten Waters like Toxapex. While it is true that a handful of Pokemon can check it and many options can win the 1v1 vs it, in practice Choice Specs Magearna can quickly grow problematic. Be it through boosted Volt Switches wearing out would-be checks like Toxapex and Slowking or through timely Tricks neutralizing special walls like Blissey and Heatran, Choice Specs Magearna is one of the best Pokemon at forcing progress each time it enters the battle.
Magearna has a plethora of different set-up variants. It has access to boosting moves such as Shift Gear, Calm Mind, and even Iron Defense that gives it a variety of options. In conjunction with the abundance of coverage options it has and the great offensive and defensive stats, this can make Magearna extremely frustrating to deal with. The most common boosting variant takes advantage of Stored Power, which gives Magearna great snowballing potential when paired with Weakness Policy, screens, its ability Soul Heart, or even ordinary free turns to set-up and increase its strength.
Overall, Magearna is arguably the best Pokemon in the metagame; there may be some counterplay present, but it is very limited and varies greatly depending upon the set it is using. Magearna imposes restrictions on teambuilding due to how oppressive the Choice Specs set is and how versatile it is overall with the boosting variants. Even if you prepare for it sufficiently, there is a possibility that it will continue to give you issues due to how effective the Choice Specs variant is at forcing progress, wearing down defensive cores, and crippling opposing Pokemon.