In the past few months fallen has been nerfed and changed quite a lot for the worse. From the massive armour breaker nerf to enraged blow damage being hit as well as rally nerfs fallen has been hit extremely hard, and while yes some of these nerfs were deserved (for example armour breaker's vulnerability was quite broken a few patches ago) I think most of the issues come from one weapon in particular, Hero. Hero especially combined with the ability to leech Battle monk's qol nodes essentially take all the risk away from fallen with points listed below.
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Air shout, half moon swipe and tempest
Usually on a more pure fallen tree you have to position well and be closer to enemies to have to hit your abilities but these battle monk nodes take all of that away for not much AP -
Walk speed
Walk speed essentially allows a good player to never get hit and it takes all the risk away from having to be low hp to make use of Fallen's high damage -
Agility and air items being very strong
Agility itself is quite strong as it makes the player insanely tanky for not much damage loss but it also stems from air items being super strong. Air items usually give tons of walk speed, damage and -spell costs for almost no downsides while giving lots of base health on top of that, and all of these factors allow Hero to do a lot of damage while being super tanky. (Pretty much taking the point out of fallen)
There has been a lot of talk lately about air items lately including #1387951219917459556 and I think some other posts that I can't find but I think this mostly affects warrior the most as seen by Hero's insane usage rate.
