We've been assured that Wizards of the Coast has put a lot of effort into balancing the current set, in an effort to avoid repeating past mistakes
One thing to keep in mind though, if you play this on your opponents turn, it will mean that your Dragon Turtle untapped before their creature does, since your next turn will be more immediate, and that means that Dragon Turtle will get to take advantage of the thing being tapped down for one attack
It is just a top up on life, a small hit of three once when you counter something
Make sure you can sacrifice or kill the Zombie Army token each turn so Dreadhorde Invasion makes a new one instead of adding a counter to an existing one
Of course, the full extent of this card is to produce four 1/1 flying tokens, which gets a lot better with Intangible Virtue or other lords
Gruul creatures are also hard to block since they will outmuscle the opposition and have in-combat mechanics like trample