Contrary to what some people believe, simplistic is not a fancy word for simple. Simplistic does not describe things that are easy to understand, deal with, or use. Those sorts of things are simply simple. However, if something is too simple—misleadingly so—then it is correctly called simplistic. An argument that glosses over or omits relevant facts and presents trite, hackneyed, slogan-like statements and sound bites is indeed simplistic.
Simple—unlike simplistic—is a term that can describe not only issues, statements, and arguments, but all manner of things observable by our senses. It can describe art, music, food, clothing—just about anything. And it is a neutral term. That is, if something is said to be simple, that can be considered either a good thing or a bad thing depending upon one’s taste. But the word is very often used positively—as encapsulated in the well-worn creative principle “Less is more”—to describe language that is straightforward and easy to understand, clothing that is not overly elaborate or excessively decorated, or architecture with clean lines. And something simple can be elegant too. To describe such things as simplistic would be an insult to their creators. And it would be ironic to use what one thinks of as a more exotic, ornamental, impressive term to describe something clear or plain and unadorned—something that is, in fact, simple.