So I was going through my masteries again recently and read the description of Deep Wounds again, and I think I realized that Zemo actually gets no benefit from it (most of the time).
To clarify - r5 Zemo owner here. My fav Skill champ.

The first piece of the mastery is duration - but Zemo’s bleeds are indefinite. N/A as a result, but everyone knew that.
The second piece, though, is the bigger kicker - it ONLY applies if Zemo’s current health is GREATER than the opponent’s current health. Considering how beefy defenders all over are these days, this doesn’t apply almost anywhere, except towards the very end of the fight, where at that point you’ve likely stacked so many INDEF bleeds that you don’t even need Deep Wounds to kick in to take down the defender. As a result, I don’t think it actually makes a difference for him.
Just thought I’d share. Let me know your thoughts!