After reading the article and looking at the photos describe some of the dangers that workers faced.

If you want an answer make sure to include the article and not just the question but, what I know from prior knowledge is that most of the danger for the workers was diseases such as malaria, yellow fever, cholera, and dengue fever. Which these diseases killed 12,000 able bodied men. (hope this answer helps you)

dynamite explosions

tropical diseases such as yellow fever

landslides that could bury workers

adapting to a different climate

the answer is they believed george iii would never give them their freedom.