Aether fiend are very long to pop when announced (have to stay on the pop point to see them poping). This are very different with Hellborne that pop allmost instantly (probably that is why most players prefers to have hellborne during infernal hordes?)
The second bad thing with Aether fiend is that lot of times they are depoping at the allapsed time of the infernal hordes vawe and not giving any aether (this again is not observed with Hellborne); a second reason to avoid Aether fiend and prefering Hellborne during Infernal hordes…
Sorry for my bad english 
geetings
Delta