I don't see those as useful since it doesn't provide the person any actual guidance or reference point. What does assertive mean? What should I exactly do? How do I do it? "Act like X" provides a well known reference point that they can use to adjust their behavior based on. They can remember all the times they've seen X do something in a similar situation and then just act like that.
If you want to make it super explicit, come up with examples. "Be bolder, for example for this project I saw you doing 80% of the work, you should get to headline the presentation and have top billing on the authors page".
"Act like X" is also potentially useful, if you make it explicit. Explicit is not "Act like a white man" (whaaaa?). Explicit: is "Act like Bob, for example do you remember when HR said he couldn't have a new screen and he insisted"?
It's just an extension of the classic "show, don't tell".