There's a few problems with tracing aside from laziness, stunting your artistic growth or even the failure to compensate your source for using their work.
1, if people notice, it brings them out of the story. They'll stop thinking about the plot and go "where did I see that image before?" or "why does this character always wear the same expression?". That's bad.
2, when you bring in sources that you don't know intimately, there's an increased chance you'll end up with meanings that you don't know about, like how tracing made Magneto the king of Spain up in >>95788418
3, it limits your options. If you've traced a character's face from an image of a person, you can only draw that character from angles of that person that you can find in GIS.
4, if you ever have to draw something without reference, there's a good chance it'll look jarring and off-model in comparison.