An iamb, I am sure you know is an unstressed syllable followed by a stressed one. Like "attempt". Shakespeare and a lot of other English-language writers liked to string five in a row (My mistress' eyes are nothing like the sun). That is what we call iambic pentameter).

A dactyl is a stressed syllable followed by two unstressed ones. Because I am lazy, I'll use Wikipedia's example:

Just for a handful of silver he left us.

("left us" is of course not a dactyl, but the ends of lines are often different from the main trend)

I just don't think the iambic rhythm is scary. Perhaps because it is so close to normal English (Dickens had a tendency to fall into using iambic meter in his normal prose sentences). Dactyls are more difficult to construct, but they are also more "unnatural", and might therefore create a spookier atmosphere. Anapests might do the same (two unstressed, then a stressed).