Okay you guys have GOT to stop saying that anything that "slows down the game" is bad, for the simple reason that it can be warped around to mean about a million different things. Technically uber can "slow down the game" by making the medic temporarily unkillable, the same way the darwins danger shield "slows down the game" by making it harder to pick the sniper with a headshot.
At one point, it was said jarate slowed down the game because it discouraged teams from pushing once they were doused. And razorback slowed down the game by making the sniper unpickable by spies. Youd think the SMG also slows down the game because its the best answer to a scout trying to rush you down. But for each one of those items, you could make the opposite case, where they speed up the game by offering a unique advantage of the other team. And as we know, finding advantages is how we push.
Also, "slowing down the game" can be GOOD in some ways. Firefights in tf2 can be seen as slower than a game like CS or CoD because they take longer to resolve. But the high ttk of tf2 is what makes it a DM game in the first place. Slowing down fights and dragging them out allows for more tactical decisions to be made. Not exactly a bad thing.
The bigger problem is classes or unlocks that slow down the game by discouraging aggression. And honestly, the Engie class is more guilty of this than any unlock released to date.