There is one major flaw in most dioramas:
A weak or non-existent story!
From the rules:
"Dioramas are story-centric, specifically built to tell a story or convey a message."
For a while, I was a diorama judge on Friday nights, and was taught by Art Gerber that this was the key. A diorama should be 60% story, 40% construction.
Without a clear story, a composition is little more than multiple models and figures on a base,and often "the more, the messier," to paraphrase Art. In essence, the model should be a "snapshot" of the description.