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Megas are fine, but they didn't expand upon the concept in Gen VII at all so the same top-of-the-line Megas remained at the top because of nothing from Gen VII really threatening them and they really only had to fear possible nerfs (dialed-back abilities, certain move combinations being illegal, etc.).
While most candidates for Z-Moves are already great and have wide coverage, it does allow for some mind-games and gimmicks (Z-Hypnosis Xurkitree, Z-Zap Cannon Espeon), but why experiment when you have Z-Draco Meteor/Superpower/etc.? Then in USUM they played favorites by giving Pokemon-specific Z-Crystals, which defeats the purpose of them being fringe options meant to surprise your opponent.
