So we know that the pro-ban side has said their piece, and I believe the anti-ban side has, too. They're both solid arguments down to their cores. But to just reiterate some of the points from the anti-ban side...
Cresselia can be checked by prominent metagame threats. Soft checks are everywhere, as every team worth its salt has at least one heavy hitter that can boost physically in some fashion. They have always been more common than boosting special sweepers. (Blissey may have had something to do with this...) Hard checks to Cress are much less common (and for good reason; Aerilate spam and the two big honking legendary threats are out there), but people do sometimes like to use Swords Dance Blue Orb Scizor or Golisopod. Those can end a Cress sweep in an instant, even if one of your members has fallen to it, and can proceed to cleanly avenge them. Scizor can even lob Toxic at it without risk, if it wants to catch a Primal Groudon on the switch.
The soft checks, on the other hand, do have to stay relatively healthy (if they get below 50%, Cress beats the majority of these), and this is something that rings true to many metagame threats. If that -ate check gets below 50%, Zygarde and Dragonite can still run over those checks. While we can also make an argument to ban these threats, I don't believe that's wise right now...
One last thing, and this is actually pretty concerning. Magearna isn't just a Cress check. It counters sweeps. And not just Specially based ones, too! I came to this realization after dwelling on the set that M'joe'ra created. Magearna is one of only two users of Heart Swap (that don't suck), and is easily the bulkier of the two. And then I realized that it doesn't just stop Xerneas, Cresselia, and Blissey sweeps. It can run Pinsirite to be mostly immune to the boosting physical sweepers, and give them a taste of their own medicine. I'm not entirely sure that this is healthy for Mix and Mega, and it needs to be looked into with greater detail...
Oh, and yes. Psychic is a real good defensive type in Mix and Mega. Less so since Altarianite is out of the picture, but Dark types are still rare.
Cresselia can be checked by prominent metagame threats. Soft checks are everywhere, as every team worth its salt has at least one heavy hitter that can boost physically in some fashion. They have always been more common than boosting special sweepers. (Blissey may have had something to do with this...) Hard checks to Cress are much less common (and for good reason; Aerilate spam and the two big honking legendary threats are out there), but people do sometimes like to use Swords Dance Blue Orb Scizor or Golisopod. Those can end a Cress sweep in an instant, even if one of your members has fallen to it, and can proceed to cleanly avenge them. Scizor can even lob Toxic at it without risk, if it wants to catch a Primal Groudon on the switch.
The soft checks, on the other hand, do have to stay relatively healthy (if they get below 50%, Cress beats the majority of these), and this is something that rings true to many metagame threats. If that -ate check gets below 50%, Zygarde and Dragonite can still run over those checks. While we can also make an argument to ban these threats, I don't believe that's wise right now...
One last thing, and this is actually pretty concerning. Magearna isn't just a Cress check. It counters sweeps. And not just Specially based ones, too! I came to this realization after dwelling on the set that M'joe'ra created. Magearna is one of only two users of Heart Swap (that don't suck), and is easily the bulkier of the two. And then I realized that it doesn't just stop Xerneas, Cresselia, and Blissey sweeps. It can run Pinsirite to be mostly immune to the boosting physical sweepers, and give them a taste of their own medicine. I'm not entirely sure that this is healthy for Mix and Mega, and it needs to be looked into with greater detail...
Oh, and yes. Psychic is a real good defensive type in Mix and Mega. Less so since Altarianite is out of the picture, but Dark types are still rare.