I can understand why teams don’t always have a Fahz. At least from my perspective, the optimal line up I always see is:
-De
-Jack
-Kait
-JD
Thus there is only one spot left for either Marcus, Fahz, COG Gear, Sarah Connor, Kat or Emile. Now this is just IMO, but Marcus, Sarah and the Reach characters aren’t super viable; Emile is probably the best as I understand (don’t own them myself) he has bleed on melee or something, but on later waves that’s a bit too high risk compared to Kait and shotguns.
This then leaves Fahz and the new COG Gear. Personally, outside of the ability to indeed kill an enemy out of bounds or something, I don’t think I would pick Fahz over the Gear. This most certainly comes from how I like to play the game, but I would much rather have the Gear who is a very strong support character (healing marks, team revive, general tank skills) and honestly IMO Invalidates Marcus, rather than just having another damage dealer.