YMMV / Equestria: Across the Multiverse

• Aluminum Christmas Trees: The double planet system the Twin Worlds Arc takes place on is actually possible to happen in real life, though two identically sized planets would be extremely rare.
• Complete Monster:
  • "Winter Wedding" arc: Princess Spring Heart/"Chrysalis" is Princess Rosie's Evil Aunt who was banished from the Isle of Pony for attempting to murder her older sister Starburst and intentionally got her infant niece lost at sea in an attempt to usurp the throne and use the stash of magical artifacts the royal family protects to conquer the world. Years later, she kidnaps her sister Winter Song and impersonates her with intent of slaughtering their entire family to continue her plans, including the now-teenager Rosie and Patch. When that plan is foiled, she attempts to slaughter the entire wedding party with her stolen jaunt tech-enhanced army, with a Dead Man's Switch that will nuke the Isle if she dies, eventually turning her homeland into a war zone. As her plans fall apart, she stops carrying about killing her own soldiers in the crossfire, and ultimately orders her sub to nuke as many places as possible out of pure spite, nearly destroying Ponyland's capital in the process. Spring Heart will destroy anyone and anything without a moment's regret to get what she desires.
  • "Monstrous 5" Arc: Eustace "Mr. E" DeMonic is the morally bankrupt CEO of Scum Labs. Obsessed with profit above all else, he subjects his workers and the public to unethical and potentially deadly working conditions and products. Taking over Crystal Prep, the student body are made into guinea pigs to be subjected to all manner of potentially deadly and untested products, with any who object disgraced, framed, or even assassinated by DeMonic's underworld connections. Making the Shadowbolts into scapegoats for the loss at the Friendship Games, they are forced to submit to horrific "product tests" under threat of their families being ruined. This ends with them horrifically ill and on death's door, a process Mr. E intended to subject 35 other innocent people to. Despite warnings of the possible consequences, Mr. E has the five subjected to cruel experimentation, resulting in them mutating into Kaiju. The end result is millions dead, all because of Mr. E's bottomless greed. Concerned with his own profit above all else, DeMonic will sink to any low to get it.
• Hilarious in Hindsight: In the "Magenta Diamond" Arc, it's revealed that universe's Diamond Tiara is a Diamond using a life long shapeshift to appear as a pony foal, which until it was discovered even she was unaware of it. It turns out in canon this is exactly what Pink Diamond was doing. Ironically, it was her alter ego Rose Quartz that arranged for Diamond Tiara/Magenta Diamond to do this.
• Moral Event Horizon:
  • The invading Flim Flam Brothers crossed it by invading other worlds out of laziness.
  • Their version of Wind Rider crossed it by showing no remorse for his actions during said invasions (like "spending time with the local mares" on conquered planets) and it's one of the reasons he's handed over to the Hegemony. His fate in being Made a Slave is as horrific as it is well-deserved.
  • Tales!Chrysalis crosses it when she orders her nuclear submarine to nuke everywhere it can before sinking when it becomes clear it's about to be destroyed.