In a software developer interview, how do you answer "what is the biggest technical challenge in the project", if there were no challenges worth mentioning?

I do like to ask a question like this from interviewees. I'm mostly probing to see how senior they are by looking at what kinds of issues they worry about.

Design for backwards compatibility, appropriate tests, getting agreement on the requirements, hitting the schedule, grasping and fitting into a larger design, navigating a complex jumble of duplicated and poorly documented APIs, dealing with difficult people, wrestling with buggy APIs from vendors, etc.

…or, you could take the opposite approach: My current job is no longer enough to challenge me, so it's time to move on. I've learned a lot there: <list>, but the challenges seem to be behind me now.

Personally, I do like to ask a question like this from interviewees. I'm mostly probing to see how senior they are by looking at what kinds of issues they worry about. Are they working on learning the trade? Dealing with people issues? Designing and incorporating feedback from others?

Where a person struggles tells a lot about what they care about and where they are in their career / life.