Unburdensome, meaning not causing any burden, has several antonyms that convey the opposite meaning. One of the most fitting antonyms is burdensome, which suggests something that is difficult, heavy, or arduous. Another antonym is onerous, meaning a task that is demanding, oppressive, and troublesome. Oppressive could also be another antonym that suggests weightiness, worry, and stress. Taxing and demanding are two more antonyms that describe tasks that require a lot of effort, energy, and time. Finally, laborious is another antonym that means something that requires a lot of hard work and strenuous effort.