You know, I didn't get that at all. Like I said earlier, I pretty much got, "demons are bad, don't do that." I'm pretty sure most would agree on that, human or not.
Except it's not just the demon. Her clansmen act like repairing the mirror itself is what's bad. Marethari and her clansmen constantly tell her things like "the mirror was lost to history for a reason, so it's better to keep it buried." (Same could be said for all Elven artifacts.) They tell her the mirror killed Tamlen (and possibly tainted Mahariel), and that by messing with the mirror she's heading down the same fate as them. (Never mind they were killed because of the taint in the mirror, not the mirror itself.) Hawke and other characters imply that the mirror itself is dangerous, Merrill is messing with powers beyond her control, she doesn't know what she's doing, you have to save her from messing with things she doesn't understand, etc. Marethari very unwillingly hands over a tool Merrill needs to keep repairing the mirror, gives it to Hawke, and a rivalmancing Hawke can withhold it from her.
That's not just about the demon. The rivalmance acts like the mirror itself is just as dangerous as communicating with a demon or practicing blood magic, and that in all situations Merrill doesn't know what she's doing, is messing with powers and technologies beyond her understanding, etc.
And yet, the characters who treat it as dangerous do so for faulty reasons. Her clansmen fear it because it killed Tamlen and tainted Marahariel. Merrill knows it's just the taint in the mirror that does it. She removes the taint, the mirror is no longer dangerous. She did extensive research on the Eluvian before she got to work, Marethari and the others didn't. They didn't even want to look into it. They just slapped huge "DANGER!" and "FORBIDDEN!" labels on it at first sight, and refused to hear her out, or do any further research themselves to see if their fear had any merit.
And of course, HAWKE knows even less about elven history, artifacts, technology, etc. than Merrill. I daresay Hawke hasn't done ANY research on such things when meeting Merrill, yet Hawke knows about the risks and dangers better than Merrill does? Hawke knows what she can handle better than she does despite knowing even less about the subject matter than she does?