Someone posted a more informed comment on Hacker News about 2 months ago. The idea is that if you drill to 5 km deep, let's say, you find very hot rock. But once you extract the heat from that rock, new heat is very slow to come because rock is a good thermal insulator. So, you'd need to produce secondary holes. Lots of them. In the end the math just doesn't work out by orders of magnitude.
Correct me if I'm wrong, but I think geothermal works where the nature has done the work for us. Where there are underground springs that turn hot and the water comes up steaming.
If you plan to drill the holes yourself, you have a lot of work to do. Drilling is expensive, and a single hole will not get you a lot of heat after the initial honey moon period.
Someone posted a more informed comment on Hacker News about 2 months ago. The idea is that if you drill to 5 km deep, let's say, you find very hot rock. But once you extract the heat from that rock, new heat is very slow to come because rock is a good thermal insulator. So, you'd need to produce secondary holes. Lots of them. In the end the math just doesn't work out by orders of magnitude.