Single celled microorganisms are sometimes called simple organisms why is this inaccurate

It is truth that most of the unicellular organisms like bacteria, are among the simplest life forms we know about it, but there are many other unicellular organisms, like amoebas, warnowiids and algae, that are far more complex. They can even have thousand of nuclei and be visible with the naked eye. Some amoeba actively hunt their pray, and some warnowiids even evolved and structure that functions like an eye; so even though unicellular, they are not simple organisms at all.