The toughest design assignment is one where the designer is dictated to by the client. I don't mean directing—guiding the process, or giving constructive criticism. I am talking about dictating the design structure and feel based on what s/he is used to seeing, or what s/he understands. When that happens, the client is not getting the full benefit of a trained creative's mind. What the client ends up with is something that fits safely within their own experience. Which is often very limiting, and not very creative.
That may be fine for the client at the moment, but the client doesn't want that either. I've witnessed situations where the client micromanaged every step of the design process. They even demanded on seeing every pencil sketch the designer created for the project. The client would have the designer work up sketches—which I would consider the weaker ideas—they "kind of" understood. And they would have the designer discard some of the stronger ideas just because the sketches didn't make much sense to them.
At the end, the client may have been happy they got what they wanted, but it didn't last for long. After looking at the final pieces for some time, they saw the designs as uninspiring and boring. Unfortunately this client could not see that the problem was the process, not the designer.