the fall in unemployment may not be productive in any way. It may be through manipulation of data, such policy as the Nazis during the 1930s.
For example, by removing women from the workforce, there are now more jobs. All the unemployed men now find it easy to get those jobs, resulting in a significant fall in unemployment. However, in the long-run, many of those jobs were more efficient being done by some of the women.
Therefore, a significant fall in unemployment may lead to a shift leftwards of the aggregate supply curve and merely be indicative of a lot of spare capacity in an economy.
Unemployment means wages stay down as there's more competition to get those jobs. As there's no incentive to work particularly hard as you are harder to replace, you may get lazy and therefore efficiency falls and there is net welfare loss.
If the unemployment is achieved by lowering a minimum wage set, then companies may hire workers instead of investing into new capital that, in the long run, would increase efficiency. While it may result in a more statically efficient economy, there is a risk of damaging it dynamically.
There's a few ideas, work on them. Remember to include at least a positive a negative, and a conclusion.