Vivien lets you play creatures from the top of the deck, she makes sizeable token blockers that mostly come with reach to keep her in play, and she can tutor out any number of sweet creature combo pieces that are regularly played
You can find the banned lists for Pioneer, Explorer, and the two formats Ill cover below by following this link
Narrowness of Effect We tend not to do too narrow of effects at mythic rare for the same reason that we watch comprehension complexity: it's important for the mythic rares to be as exciting to players as possible (within the confines of what's good for the game)
Even if there's nothing tapped for her to kill, her abilities make combat a nightmare, as she can create a blocker or grant first strike to an existing creature
The other interesting aspect of coven is that it demands you commit to the board fairly significantly to see any results
Sideboards in MTG can take your deckbuilding to the next level