The only downside for Zareth is if you want to use its second ability, you have to somehow get it into your hand, which can be done with ninjutsu cards like Prosperous Thief, trading them back and forth from your hand
Sample Strategy: Dimir Cowabunga Shell (Standard) Typical Turn Sequence: Turn 1: Drop an evasive threat like Siren Stormtamer
Today Mythics are the most tournament playable rarity, with Mythics seeing twice as much play as Rares
Having to hold mana up is potentially more annoying than being a free activation, but you do end up getting both hexproof and indestructible, making it worth the mana
Reveal the player you chose: Choose new targets for target spell or ability if it's controlled by the chosen player and if it targets you or a permanent you control
Dragon support cards help your dragon deck run smoother, fix your mana or speed up your plays