Thing is, Lopunny-mega forces out the setup sweepers that fear FakeSpeed (even when it doesn't actually run it), including Ghosts, thanks to Scrappy.
Due to that, if predicted correctly, Lopunny can actually use a Swords Dance when the opponent switches, and then break through some of its checks with HJK and sometimes its useful 4th move slot.
Now, let's see what it can do after a SD :
+1 252 Atk Lopunny-Mega Ice Punch vs. 252 HP / 216+ Def Landorus-Therian: 404-476 (105.7 - 124.6%) -- guaranteed OHKO
+2 252 Atk Lopunny-Mega Extreme Speed vs. 252 HP / 252+ Def Toxapex: 144-171 (47.3 - 56.2%) -- 82.4% chance to 2HKO
If Lopunny has Encore (and Scald doesn't burn), it wins.
+2 252 Atk Lopunny-Mega High Jump Kick vs. 0 HP / 0 Def Diancie-Mega: 405-477 (168 - 197.9%) -- guaranteed OHKO
+2 252 Atk Lopunny-Mega High Jump Kick vs. 252 HP / 0 Def Magearna: 390-459 (107.1 - 126%) -- guaranteed OHKO
+2 252 Atk Lopunny-Mega High Jump Kick vs. 252 HP / 0 Def Mawile-Mega: 363-427 (119.4 - 140.4%) -- guaranteed OHKO
+2 252 Atk Lopunny-Mega High Jump Kick vs. 248 HP / 200+ Def Rotom-Wash: 313-370 (103.3 - 122.1%) -- guaranteed OHKO
Oh, and covering Drampa's Grandpa list of Lopunny-mega's 1v1 matchups :
Thundurus, Medicham-mega, Pidgeot-mega, Charizard-mega Y are all OHKOed by Extremespeed at +2.
Offensive Altaria-mega lacking Fake Out and offensive Charizard-mega X are also both OHKOed by +2 Espeed after Stealth Rock. (87.5% of the time for Altaria-mega)
What else? Well, having a partner like Magnezone helps it, as it can trap and kill Celesteela (unless Shed Shell), and Ferrothorn, in case it can't risk HJK against King's Shield.
But anyway, Lopunny-mega itself doesn't require much team support to do its job, even though it enjoys slow U-Turns and Volt Switches when it runs Fake Out.
But that's not everything.
Encore turns out to be really effective on it, as it can allow Lopunny-mega to use even more Swords Dances, and thus removing even more of its checks.
One of its main points of selling is that FakeSpeed invalidates some setup sweepers, but it turns out that Lopunny-mega can be a threat to both offensive and defensive playstyles due to Swords Dance and Encore.
Drampa's Grandpa already calculated Lopunny-mega's 1v1 matchups, so i have nothing more to say about it.
Honestly, i think suspecting the Lopunnite may be a good idea, since if Lopunny-mega really isn't unhealthy, then it will just be kept. (it's indeed not much of an argument, but the subject of Lopunny-mega would be more heavily discussed that way)
Oh, also i did this with my free time