Sentence:  Economics is a messy discipline: too fluid to be a science, too rigorous to be an art. Perhaps it is fitting that economists' most-used metric, gross domestic product (GDP), is a tangle too....But as a foundation for analysis it is highly subjective: it rests on <WSD>difficult</WSD> decisions about what counts as a territory, what counts as output and how to value it. Indeed, economists are still tweaking it. 
Meaning of the word marked with <WSD> tags:  Hard, not easy, requiring much effort.