The Fallen set is a pretty good armor, it give resist to 2 of the worst statuses (poison and fire) and give a universal Med ASI increase. Combo this with the SSB and you get a universal ASI of Ultra. I would not suggest this set for new players though as some of the worst enemies are fiends for first time player go for Valkyrie or Dread Skelly to deal with them. Why? The fallen set has a VH universal penalty to fiends so unless you want to pay for fiend UVs and use your pet perk to help cancel this, if you can do that then this set would be useful for handling fire themed fiends (fire/shadow is a rarity among shadow armors with only one other set doing this).
The other reason to get this set is because you already have VH ASI and using Vog or some other VH ASI armor is redundant.
It is also the second best status immunity set for C42 (after Dragon scale set of course) for those too cheap to get a poison UVed Vog set.
All in all its a pretty good set, not a truly horrible set but it does have some downsides.
Great for undead
Good for ASI hybrid
Bad for fiends
The funny thing about it is the pairing of shadow protection with fiend penalty. So, what, I'm supposed to use it only against undead? Fortunately damage protection on armor isn't as big a deal as status protection, and the status protection is nice.
Fallen isn't terrible armor. Neither is Deadly Virulisk. They just seem bad in comparison to the truly good armors.