Can i just go on record allready and say that i want Neinhart to die horribly, after only one panel of his presence? I just don't like characters who have the powers of "Bring back bad memories of dead family/friends", because there's no way it would do any more than mildly confuse the victim for a few seconds, before the bad guy is stabbed repeatedly.
Especially since this guy is really misusing his powers. He could bring back plenty of other people in their pasts to curbstomp them. But no, he plays the Mindf*ck game. Make a copy of Zeref from 3 minutes ago, and win instantly.
Well as I said before I seriously doubt that these are just harmless projections to confuse someone. That Simon used his magic, means he can create 'real' images and their magic. In fact this bears a strong resemblance to Gemini's magic where they can copy a person, their memories and their magic. Neinhart's however seems to be on a larger scale.
Yes magic types like these should be more elaborated to specify its limits, however Hiro only does that with the abilities of characters that appear more regularly. I can think of several ways how Neinhart's ability is restricted, though those are merely possibilities. I think it's safe to assume though it has the same limit as Gemini, namely he can't create a person and that person's magic if the magic is too powerful. So it's rather unreasonable to think he can copy Zeref, who is supposed to be even far stronger than himself and Erza and the others don't know how powerful Zeref exactly is, that too is a possible limit of Neinhart's magic. Erza knew how Simon looked when he died and what kind of magic he used, so Neinhart could copy that, however she doesn't really know much about Zeref.
By all means his magic will have limits, but yeah I doubt that Hiro will elaborate much on that unless he has future plans for Neinhart.