Challenging users in the correct manner will ultimately lead to more engaged, informed, and self-sufficient users. Informed users have a better idea of what they want and can better articulate why they want it. Most importantly, a user who is engaged with a subject is more willing and able to grow with it.
In fact, a complex UI can sometimes be exactly what the user needs. For example, a dashboard that displays visualizations of large data sets can only ever be complex because its purpose is complex.
