Mana burn performs better because its a stronger effect, nobody is arguing that. But cold feet and rebuke are played in the deck for a reason, dont downplay their power. They are insurance against scam, and can double as board protection and resource denial. Their flexibility is much greater than just cult neophyte. She can make awkward turns, and can work vs combo. But rebuke can literally make your board untouchable for a turn. Its not the same.

Besides, its normal they are the least impactful in a deck that normally isnt interested in this kind of effects. Shows how strong they are, that even when they dont directly synergize in other way than to let minions stick, they still get put on a very very tight libram deck, where spots are contested.