Depends on the hard science and depends on what you think of as hard. Even something like biology can go from soft to hard. The hardest hard science is physics, next would be chemistry (though there is overlap between the two because chemistry is built on physical processes). I think physics majors have it harder than most engineering majors, but EE, chemE, mechE, and aero all rival physics in terms of difficulty.
Engineering tends to have a higher quantity of homework and projects than a hard science, but hard science tends to require a greater understanding of mathematical analysis and underlying processes. For example, EEs have to do a lot of circuit stuff, which physics majors only dabble in. But physics majors learn all about quantum mechanics from the ground up while EEs are handed some relevant formulae (your school may differ).