From what I understand as someone who is not great but watched a few of the best runs, not everyone is convinced that Fury 2 is actually any faster than Fury 1. At least, that is what I heard when watching one of Corvus’ runs, who is currently 2nd on the leaderboard. While vow of fury makes enemies spawn faster, 2 is thought to not make a difference over 1, so some runners don’t use it.