The danger is being overly optimistic about how long it takes to do that.

Things don't 'fall' normally around small cosmic bodies. The local gravity is so low that any lateral velocity has an exaggerated effect. The behavior of objects around asteroids is counter-intuitive, if not absolutely chaotic.

It would have devastating consequences if it hit. There is the serious question of whether, if it is headed toward impact, we will know enough to make a timely decision.

We'll go many years with basically no additional information on where it's headed.

That (will be) an impossible task, I'll tell you right now.

Because of this low gravity, we B612'ers talk about 'docking with' a near Earth object and not 'landing on' one.