The biggest difficultly of "engineering" comes from what jobs you are being trained to do. (don't get me wrong, the required math/physics courses are difficult, but they are usually not what kills people in an engineering major) This is easiest for me to explain in terms of comparing it to biology, since I did both:
An undergrad Biology education (in general) prepares you to be a technician...someone who is going to perform a set of tasks (gels/pcr/cultures/isolations/etc.) at the request of a PI.
Undergrad engineering programs (bio/chem/ee/whatever), on the other hand, prepare you to be a problem-solver...someone who can use their foundation of technical knowledge to generate a novel solution to a problem in their given field. Because of this distinction, many jobs equate a BS in engineering to a MS in other fields when it comes to their pay scale.
Engineering requires critical thinking, reasoning, and problem solving ability...traits that are much more innate and difficult to "learn" than what is required of many other majors. Therefore it *does not* reward (and acually has a tendency to weed out) the super-hard-working traditional premed mentality/work-ethic that can be applied succesfully to other non-engineering tasks like memorizing the kreb's cycle. Becuase of this, people who fail to have/obtain these necessary skills generally will be frustrated as they try in vain to fix the problem by spending their lives in the library.
On the other hand, people who are just naturally good at critical-thinking/reasoning/problem-solving often find engineering classes much easier than traditional bio type classes because they often don't require the brute-force studying/memorizing like many bio classes do.
This all being said, the particular difficulty of BIOengineering comes from the interdisciplinary nature of the subject...knowledge of bio, chem, physics, medicine, physiology, mechanical engineering, electrical engineering, computer science all must be integrated in order to address problems that span all of these fields.
Hope this helps! 🙂