Well done for trying to use this word. It's a good way to learn. Your sentence is not a bad effort but not very natural, in my opinion. If you look at the example sentences at the link below (those which use "understated" as an adjective), then you will notice that it is often used to describe an objective quality of a thing. In these sentences, look at the kind of nouns it is used to describe e.g. impact, texture, integrity. http://fraze.it/n_search.jsp?hardm=1&t=0&l=0&p=2&q=understated

Grammatically it is correct. Here is an example using "understatement": Person A: The universe is big. Person B: That is an understatement. It's enormous! To understate something would mean to lessen the reality. Though it does not necessarily mean that a statement is factually incorrect. It can often mean that the word choice of the statement is more restrained or moderate.