In Mermaid mythology, are they actually dangerous and man-eating?

Yes and No, there are numerous legends and actual folk tales that describe humans marrying a mermaid, often the mermaid would have some article of clothing like a cap called a Cohuleen Driuth or a dress that was somehow linked to her and a human could gain control over her by possessing the clothing.This is seen in Irish tales and the creatures were caled Merrow, it is likely one of these tales that Disney was inspired from.There is a medieval coat of arms with the emblem of a mermaid on it and the family claims that they are descended from a ancestral dalliance with a mermaid. You are correct in a way that the mermaid may represent the capricious, unrepredictable nature of both women and the sea, for yes the mermaid archetype is often not always pleasant.You have to keep in mind that all myths are nebulous and duality presents itself in everything.However it also simply could be argued that sailors often wreck their ships because they are too busy trying to get a good look at the bare breasted mermaid!