The fact that they have a better A.I than the cabal or Vex, it makes them nearly impossible to beat even with the best gear. You got the bullets that track you, snipers with almost pinpoint accuracy and kill you in almost one shot, you have the commanders with the spammed teleporting which makes them EVEN HARDER TO KILL, along with the fast moving sword fighters. The Fallen are the strongest A.I and no one can argue with that.
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The bullets which track me, I use in cover. Snipers with almost pinpoint accuracy? The Vex hit every time. The Fallen do not. Commanders with the spammed teleports? Predictable and normally armed with shotguns. Will be no issue come HoW. Fast moving sword bearers? The Vex Teleport faster and further then they do. Simply have a lookout for them.