If you’re describing something simple, minimal, functional, and well-designed, you don’t mean to use the word ‘simplistic’. The Oxford English Dictionary defines simplistic as:
‘Treating complex issues and problems as if they were much simpler than they really are.’
Simple has neutral connotation, but simplistic is negative and critical. Here’s a correct use of word simplistic, from the New York Times:
“Today we are threatened by a new simplistic approach that reduces all the problems in the world to the struggle against terrorism,” Mr. Védrine said during a lengthy interview on France’s Inter radio. “This is not well thought out.”
Védrine is saying that the US’s approach is oversimplified - that it lacks essential elements and ideas.
I suspect this mistake stems from a false parallel with minimal. Minimal and minimalistic have the same connotation and similar meanings, whereas simple and simplistic are very different. When writing approvingly, simplistic is probably not the word you’re looking for.
I hope that this rant steers people away from the avoidable mistake of misusing simplistic.
You probably shouldn’t use minimalistic either. It’s barely a word, not worthy of the Oxford English Dictionary or Merriam-Webster, only appearing in the expansive Collins Dictionary of dictionary.com fame. It only started showing up in the 1950s, and then mostly to refer to minimalistic art, not minimal things in general. Minimalistic is a lot like ‘utilize’ – a word that adds letters but not meaning.