The effects of sinkholes vary based on where and how they were formed.
When naturally formed on land, they will change the topography of the surroundings, cause nothing but minor issues, if any issue at all.
When formed on highly populated areas, though, they can cause a lot of damage to humans and properties in the region. Damage caused to the base of a building might be a very serious problem.
Toxic chemicals can also come up through sinkholes.