Okay, four posts on Magearna so I figured I should respond to them. These reasons are I (and I believe some others) do not believe Magearna should be suspected.
First of all, people seem to be thinking that versatility = brokenness which is
definitely not the case. Magearna's pool of checks and counters stay roughly the same no matter what set, because the attacking move Magearna runs aren't that different from set to set (and yes you theoretically could run subpar moves like Ice Beam and Energy Ball but you are going to be losing to many more Pokemon and using these moves in your argument is not helping your case). Also these posts are implying that it is super easy for Magearna to set up which it is obviously untrue; it's pretty easy to wear down with the combination of hazards + offensive pressure and not to mention the fact that it's an important switch-in to moves for both Steel and Fairy. Setting up also is risky when the reward is probably not that great (Calm Mind is easy to revenge kill and Shift Gear is easy to live a hit from). Before breaking down other people's arguments, I'd like to give the reasons why I think Magearna is not a broken threat in this meta. Magearna does not do any job absurdly well; as a wallbreaker and cleaner it's stupidly slow and even at +2 is outsped by some common threats like +1 M-Sharpedo, +1 Scolipede, Scarf Terrakion, Scarf Greninja, Scarf Latios, Sand Rush Excadrill, and Surge Surfer Raichu-A. Shift Gear's main problem, however, is how easy it is to wall or live a hit from. Common walls in Mono right now can easily deal with it (e.g. Heatran, Chansey, M-Scizor, Celesteela, Zapdos, Mew) and the fact that Fleur Cannon lowers Special Attack means that it's hard to consistently pressure walls with your STAB move, definitely a downside. Specs and AV are easily revenged (which means the whole "Magearna kills offense" argument is false but I'll get into that later) and OTR is just a worse Shift Gear, not sure why that is being brought up as much as it is. Anyway, onto the arguments:
To start with...
Mega Diancie or Mega Gardevoir would cause Fairy to only skew further towards having the advantage in theoretically-disadvantageous match ups.
If it's not "broken" now, think further down the road when Mega Diancie and Gardevoir get released and open up an enormous realm of possibilities for Fairy types.
We are talking about the metagame
right now, so bringing up future Mega releases that we have no idea are going to happen is neither improving the validity of your argument nor making an actual decent argument.
There's just too much this mon(ster) can do. From Jo' Z's previous mentions, Magearna could run quite a list of moves/sets depending on the type; Fleur Cannon/Flash Cannon and Ice Beam are common moves that it can run, however momentum switching moves such as Volt Switch and Trick Room can be beneficial as well: both Steel and Fairy have reliable U-turners such as M-Scizor and Tapu Koko that always do their best to keep the power in the hands of the player that wields them. Trick Room for Slower Fairy (primarily) and Steel teams also provide a huge benefit, especially considering the teammates that could benefit from it.
magearna obliterates many offensive and some balance teams with shift gear/tr and twinkle tackle+fluer cannon. Its only flaw is that it only has 4 moves and can't hit everything supereffective, but does that really matter when you have aura sphere, tbolt, z fluer, ice beam and 5 other mons to take out anything that could possibly tank your mag set at +1. Any type that doesn't resist fairy is threatened by mag sweep especially the ones that tend to be offensive (fighting, flying, dark, ice, electric).
As I've stated before, versatility isn't necessarily indicative of anything does not mean Magearna is broken. This is definitely not a good main argument. Magearna has a very similar pool of checks no matter which set it is running, it is not like you have to dedicate a Choice Specs check and an AV check and a Shift Gear check because they all essentially have the same coverage: Fairy / Fighting / Electric / (maybe) Steel. This is not that hard to check, as I have said previously. Don't want to parrot myself too much so I'll just leave that at that, but this argument is just plain unconvincing.
Also, incidentally, this might make the Dragon vs Steel matchup a tad more bearable for Dragon teams, instead of getting swept by Shift Gear Magearna once your Garchomp is out.
Any type that doesn't resist fairy is threatened by mag sweep especially the ones that tend to be offensive (fighting, flying, dark, ice, electric).
Overall, it's scary for any type to face and I think it deserves the spot for most threatening mon in Monotype.
No, Magearna is not responsible for how Dragon, Ice, Dark, and Fighting aren't good types right now. People tend to point fingers at Magearna as it happens to be a good mon on both types that threaten this list of types but it's the entirety of these types, not Magearna itself, holding them back. Magearna exploits the general weakness of many of these types to powerful Fairy STAB but that does not make it banworthy. Face it, these matchups would not be much better without Magearna.
Also, many seem to believe that Magearna is a huge threat to every type when it's definitely not, I'm just going to say a few off the top of my head: Steel, Normal, Ground, Water, Electric, Flying, Psychic, Fire. Keep in mind that all of these are top-tier or decent types in this metagame.
Closing thoughts on Magearna (use this as your tl;dr): Magearna is definitely a good Pokemon, but not suspect-worthy. It capitalizes on weaknesses of subpar or bad types and has weaknesses no matter which set it runs. Pretty much every type has one or two checks to it and can succesfully revenge kill it. Soul-Heart, while a good ability, is basically special Moxie which people are over-dramatizing in my opinion; Magearna has a low speed tier and relies on set up to actually sweep the opposing team (unless it is super slow like Water Stall). Except wait! Don't quote me on saying that Magearna invalidates stall which it does not--the format of Monotype makes it extremely hard to be effective. Balanced or offensive, teams have checks to Magearna that aren't uncommon or bad. The arguments for suspecting Magearna are the same and really are not that great, and I have yet to be convinced that Magearna is even remotely deserving of a suspect.
Sorry for the long post!