Well there is not to many bad things to say but there are a couple. The first problem is the costs of these systems. They run 5-10k in equipment and another 5-10k installation costs. Most of this is from drilling or digging involved to put the piping in the ground needed to get the desired temperature both in the summer and winter for your house. Because of these costs, and the marginal savings a pay back period of 10-15 years is very common. Other problems are pumps burning out. This isn’t a huge problem but will costs a little bit of money to fix when it happens. I have not heard of any environmental problems cause by heating up or cooling off the earth around the piping in the ground. This is due to the large amount of pipe that gets buried, the transfer of energy is very low spread of that distance of pipe.
One last thought is if everyone switch to geothermal heating, there would be a need for a lot of installers for the initial rush of orders, but over a long time period less work, maintenance would lead to less jobs in the energy sector. Everyone can safely agree that if there were no more need for power plants, or even only have the current demand there would be a lot of people out of work.