Difficulty depends on the person. Literature is not hard for someone who’s good at literature. There are lots of applicants who find maths and sciences much more difficult than literature.

It might seem more impressive if you’re a STEM applicant and got 800 on Lit (given you’ve already achieved 800s on other science/maths SAT IIs)

Also, It is true, I think, in general—about 1% of those who sit SAT Lit get 800, as compared to 12% of Physics candidates and 18% of Maths II candidates. But, like Louisa said, how good you are at each subject also obviously matters. (You don’t need to be good at literary analysis for Literature, just at reading comprehension. It’s basically SAT Reading v2, if you took the old test.