As with so many things, it depends on the person- and which STEM subject and which humanities subject. Not to mention how you define each: in the US, “STEM” includes the social sciences, whereas in Europe they are usually considered humanities. And not to mention what you mean by ‘challenging’!
STEM subjects are harder to get high GPAs in (cue CC discussion on why…), but beyond that it’s a pretty meaningless conversation. Is anthropology harder to master than history? Is organic chemistry inherently harder than ancient Greek?
The only thing that I would say is not to close yourself off too early- classes in college are just different than HS, and people grow and change. I know a fair few students who have been very surprised to find that they enjoy subjects on the other side of the STEM/Humanities divide- perhaps not as a major, but as part of their college experience.
As the late, great Maria Montessori said, the best learning experiences are ‘challenging but achievable’.