simple, simplistic

Simple means “easy,” “plain,” or “naive.”

Simplistic is a pejorative adjective meaning “overly simple, facile.”

Careful writers reserve simplistic for its pejorative use and refuse to use it to mean “simple.”

Also, note that simplistic already has the notion of "too" in its meaning. It's therefore tautologous to refer to something as "too simplistic" or "overly simplistic."

Example: We expected the exercise to be simple, but his instructions were so simplistic that it ended up being difficult.