I dont know of another deck that people are supposed to be more afraid of, but I expect a diverse metagame where this is a player, and Id be surprised if Mono-Green Devotion is the most-played deck
Elspeth, Sun's Champion: One I don't really want to remove but she really just says destroy all creatures power 4 or greater for 6 mana, and most of the time it hits Sram unless he has indestructible
The first spoiled deck appears to be an Abzan deck named 'Corrupting Influence', with Ixhel, Descendent of Atraxa in the face Commander's chair
Due to its limited distribution, the Rebecca OP05-091 card is rare and commands a high price in the secondary market
Throw in some Infusion utility in the form of Poisoners Apprentice, Foolish Fate, and Thornfist Striker, and this Witherbloom deck was an absolute steam train once it reached turn three onwards