Being able to flashback a Mana Leak or a Fatal Push can absolutely ruin your opponents strategy
In the world of green midrange or ramp decks, landing a Harmonize often means sustaining the pressure and avoiding running out of gas
If it's a creature card, you lose life equal to its mana value
You now have enough mana to repeat the process, making another two goblins with Pashalik Mons' ability, making mana with Mana Echoes, sacrificing one to Skirk Prospector, activating Pashalik Mons, and so on and so forth until your opponents are all dead
And if we cant shut down the card draw altogether, well at least find ways to punish it
Whenever a white creature you control attacks, you gain 1 life