We shouldn’t use em dashes at the start of a list to replace a colon, nor in lists to replace semicolons. We should also be careful not to overuse the em dash in our writing because the other forms of punctuation listed are the standard accepted forms and should be used, mostly. If you need the reader to pay closer attention to a particular point, however, then using an em dash in place of the other punctuation is a good way to grab their attention.