People often confuse hard stuff with complex stuff. Most complex stuff is hard but not all hard things are complex.
A lot of hard stuff is not complex. Getting in shape is not complex, it is just hard. Building a house by standard blueprint is not complex, it is just hard. Deploying a lot of servers in a cluster for product build for it and is well tested and documented is not complex, it is just hard.
Complex stuff means, that it is an emerging practice, product, something unexplored. Curing most cancers is complex. Building a house using completely new materials never used is complex. Migrating large old badly documented system to new badly documented system is complex.
Juniors can deal with easy stuff. Mediors with hard stuff. And Seniors with complex stuff.
If you missmatch level of skill and experience with the task, you set yourself or the person you manage up for failure.