It should be worth mentioning that the council have decided
Mawilite and Beedrillite should be
banned from non-native users immediately as they release. This decision was made a month ago, following the announcement of the mega stone releases, as we have all agreed these Mega Stones will only aid towards the over-centralisation of offense, giving it all the utility it needs to make the playstyle extremely potent, and not letting defensive teams compete. I'll break each down into their own arguments.
*Technically, these Mega Stones were already banned when the banlist was carried over to Gen 7; however, for the sake of analysing their current place within the metagame, a conclusion on their status seems more appropriate now.
Mawilite +20 Atk / +40 Def / +40 SpD | Huge Power
Evidently, the stat increase present here isn't what makes it banworthy, it's an inferior Sablenite if solely considering that. Huge Power simply makes Mawilite overpowered in the selection of offensive stones, giving offensive presences over twice the amount of physical power, while aiding their longevity with the addition of 40 base points to each defense. Let's take a look at Pheromosa per se. At the moment it runs Lucarionite mainly, because that's its best option; however, with access to Mawilite, its power skyrockets.
Just a few, not so relevant calcs, to prove power rather than showcase how it performs against various threats.
252 Atk Huge Power Pheromosa High Jump Kick vs. 248 HP / 8 Def Groudon-Primal: 322-379 (79.9 - 94%) -- guaranteed 2HKO
252 Atk Huge Power Pheromosa High Jump Kick vs. 0 HP / 4 Def Arceus-Ground: 415-490 (108.9 - 128.6%) -- guaranteed OHKO
252 Atk Huge Power Pheromosa High Jump Kick vs. 248 HP / 0 Def Zygarde: 415-489 (99 - 116.7%) -- 93.8% chance to OHKO
This is only considering one user of the stone, while more powerful slower Pokémon can take advantage of the bulk increase and be quite effective in removing threats.
Beedrillite +60 Atk / -30 SpA / +70 Spe | Adaptability
Alternatively, for slow attackers that don't have the necessary bulk to survive hits even after the increase, Beedrillite may be a more appealing option. You rid of the marvellous ability in Huge Power for a fine ability in Adaptability, hence losing some attacking power, but easily making up for it with the large increase of Speed and a still massive attacking stat. The Attack increase is already stronger than any previously existing Mega Stones, compared to Lucarionite's +35 and Adaptability, this Mega Stone is too powerful for the metagame. Consider how potent Lucarionite currently is on physical attackers, then double its stat increases. This is basically that. This isn't as overtly broken as the first stone, but it is still easily banworthy.
If you want these back, you better wait for Game Freak to release a Fur Coat stone :P It's unfortunate that the two stones released first are two of the three that we plan to ban on release (the other being Medichamite), but it should mean that later on the metagame won't lose activity, due to the constant addition of other stones.
Tagging
The Immortal to ensure Mawilite and Beedrillite are banned from non-native users when they're added.
Also, we agreed on the next suspect test, which will be Dragonite
. This should be coming up shortly.