Eric Musco took on the official forums, to report that Nightmare loot are not a guaranteed drop from nightmare operations.
I have a few updates to pass on regarding the current issues with Nightmare Loot Drops. First, we discovered there is a bug with the percentage chance of a 224 rated item dropping in any of our Nightmare Operations. We plan to address this when we release Chapter 10 (Game Update 4.1) scheduled in February. If we are able to address it sooner, I’ll pass along an update.
Aside from that, we realized that we need to make a correction to our Operations and Flashpoints in Fallen Empire blog. Currently it states the following:
Nightmare Mode will also be available for all the Operations that had it previously, and will reward the same as the highlighted Hard Mode, as well as the unique mounts and titles available currently.
This statement is in error and needs to be corrected. When running a Nightmare Operation, there is a chance that a 224 item can drop, but it is not a guaranteed drop like the highlighted Hard Mode is. Nightmare Mode Operations will still drop their unique mounts and titles, and once fixed there is a chance to get 224 gear from them. It is by design that the best and most consistent way to gear up in Operations is to run the highlighted Hard Mode each week. Although they did certainly want to make sure there was a chance for greater reward from Nightmare!
We apologize for any confusion and will make sure that the blog is updated to reflect the correct information. I will pass on any further updates as I have them.