At 22.5 damage per turn before buffs, plus 1 round downtime, the unbuffed Heart of Vengeance does very low damage for its cost. By the time the Heart of Vengeance Druid can power up against Aggro, the player either will lose or would have been better suited with anything else that can make the opponent play long enough that they have issues with card draw. That being said, this Monkey is still alright if enough bloons are weakened and leaked safely.
In Control mirror matchups, Spike-o-Pult and Wizard Monkeys have far less restrictive requirements for scaling, start with more base damage per turn, and in the case of Spike-o-Pult, synergizes well with Try this!.
Rather, the Heart of Vengeance Druid stands out against a constant flow of bloons when the life loss is buffered by Jungle's Bounty Druid's rapid life gains. Regardless, its scaling is still more unreliable than the Wizard while having 1 round downtime.