Everyone has experienced the frustration of losing Game 1 to a card that you can't interact with
It won't trigger again while the ability is on the stack, but if the ability is countered and Dark Depths is still on the battlefield with no ice counters on it, it will trigger again immediately
Where Zombies starts to differ from Vampires is in the number of lords
In fact, its so rare that the few players who have it may not want to play with it, lest its value decrease
Then, you will end up generating 12 Gnome tokens instead of six with Thunderhulks triggered ability
Murmuring Bosk, for example, pairs nicely with fetch lands that look for a Forest, while Tournament Grounds is perfect in decks heavily centered on Knights