Underused means something is not used as much as it could be or should be — the capacity is there and it is going to waste. The complaint inside the word is aimed at the situation, not at the thing itself: an underused library is a perfectly good library that too few people visit; underused land is land sitting empty; an underused railway line carries few passengers. It is very often used about people and their abilities, and there it stings gently: an underused employee has skills the job never asks for, and 'her talents are badly underused in that role' is a sympathetic thing to say about someone. Sports commentators use it constantly about players kept on the bench. It is a mild, reasonable-sounding, slightly official word — councils, managers, and reports love it, because it points at waste without accusing anyone directly. You will also meet 'underutilised', which means exactly the same thing but sounds heavier and more corporate; 'underused' is the plainer, better choice. It usually appears with an adverb of degree: 'badly', 'seriously', 'chronically', 'sadly' underused.