If you’re the victim of scrying, there’s a feeling of powerlessness that comes with the experience. When a BBEG points their magic mirror your way, they suddenly know where you are, what your plans are, and (most likely) how to defeat you. That’s not a good feeling. It engenders paranoia, simmering resentment, and a deep-seated desire for revenge. And while you may want your players to hate the villains, you don’t want them to hate
you.
It's not just scrying either. If you do decide to make power-plays with magic — unlimited counterspells, teleporting your villains, or otherwise using abilities that players can’t reasonably counter — think long and hard about the impact on the game. Players can scent a whiff of unfair GM-fiat a mile away. Dressing it up as “some of the casters in this setting are more powerful than you” is fooling exactly no one. Because railroading is a dirty word, even if you dress up in the rules.
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Comic for illustrative purposes.)