dialogue on a slide

That's a great question! When you say "dialogue callouts" I'm assuming you mean things like speech bubbles. If so, then I fall solidly into "sometimes."

I don't think they inherently look simplistic. And I've had multiple projects where they were the right choice. But sometimes speech bubbles don't entirely fit the look and feel of the rest of the course design. So in cases like that, I'll use more subtle signifiers that the content is dialogue, like putting the text in a simple box (like I did in this project) or putting it to the side of an onscreen character enclosed in quotation marks.

Another thing to try is to adjust the visual design of the speech bubbles you use. The default ones may not match the rest of your course, so changing up the shape, color, borders, or shadows on them may help. And sometimes you might want to build your speech bubble shapes from scratch instead of using the default ones. That's thankfully quite easy in Storyline or PowerPoint.

If you're trying to decide if speech bubbles are or aren't a fit for your project, it also might be helpful to browse through our E-Learning Examples and Downloads pages to see if any of the projects there give you design ideas.