It affects your team only. Otherwise, the sentence below doesn't make sense.

"Ouch. That hurts. But do the Tactics make up for it? Survey says... you bet."

If it affects the enemy team only, why does the Tactics had to "make up for it"

There's also no rules about if it's negative negative, like Thaddius Brood, it is enemy team only. There just wasn't a summoner "yet" that would affect your team negatively because debilitations hasn't introduced yet so an ability negatively affecting your team didn't exist until now.

But it's just a theory until we see the card release.