We didn't give enough credit to this person, assuming they wouldn't be as effective or capable as they turned out to be.

We didn't fully grasp the scope of this difficult situation. It's been much more challenging than we initially assumed. We're still finding our way through the complications that have arisen. The project has presented obstacles we didn't anticipate, and we're working to address them as they come up. We've learned that our earlier expectations were far too optimistic. We need to continue adapting our approach as we uncover new layers of this problem. It's a humbling experience that's teaching us to be more careful in our assessments going forward. The reality has proven to be much more demanding than our initial projections suggested.