I think scientists can monitor things and have a fairly good idea something is going to happen, they just can't say exactly when. Take for example Mt St Helens. They knew it was going to erupt and could even be fairly sure how much damage would be done, but they couldn't say exactly when it would blow. I think the same is true for earthquakes. We know enough about the precursors, scientists can be fairly sure one will happen in a particular place, but not when. So they can be predicted, but it's not always precise enough to be able to prevent the death and injuries related to them.
When it comes to the consequences, I think that can be predicted more accurately. Scientists know how much shaking buildings can withstand and where people would be at certain times of the day so they can estimate casualties and property damage. If they know the type of volcano and what will come out of it, they can determine how much immediate damage there will be and how much of a larger area will be affected by the ash cloud, etc. But then again, there are always unknowns scientists may not have taken into account or even knew about.