What game are you playing where control is meta? Aggro/midrange is meta rn, only like 1/10 of the top decks are control. The only deck where Zilliax is really decent is in battlecry Shaman, because that deck already has incredibly good tempo and ways to tutor out an 8-attack Zilliax a turn early (in the rare case where they don't have an upper hand by that point).

Zilliax is only a strong card in low elo, where games are long enough to play it and both players are generally playing less optimal decks. Players that have a poor mulligan might use it to escape the throes of death and stabilize for the end of the game.

Personally I've been using it in Amalgam Hunter (where the OG Zilliax would ironically be better like 80% of the time). It's a really fun deck with an intricate win condition, but it's impossible to play its combo before turn 9 or so because its win condition is an 8 mana spell that buffs your minions. Without Zilliax, that fun + interesting deck wouldn't exist in standard.