Disadvantages:
The most important disadvantage is absolutely the geological problem. The heat source is mostly close to volcanic activity of some sort.
Chemicals are byproducts of the production electricity with hot ground water. Some geothermal plants use a lot of water and it needs to go somewhere its after use. Some of the poluting chemicals in that water and steam are sulfur, mercury, hydrogen sulfide, arsenic, ammonia.
Earth is a changing creature. A drilled hole in the ground could supply thousands of homes heat and one earthquake could change that in a second. It can also change gradually over time.
Location, location, location. That is certainly true of geothermal energy. It cannot be transported over vast distances. If used to heat up houses or for hot tap water it is only the quality of the pipe that delivers the water that determines how far it can go and if it will be of any use when it arrives. If the heat is used for electrical production it helps to have plants close so the energy loss is not too great.