What are the down sides of a geothermal heating/cooling system? And with radiant heating are there any down sides? What happens is the tubing breaks/

What type of system are you looking to install?

On the heating side it is similar to a heat pump in that it works best when it is not overly cold out as the air is not being super heated it is only being warmed.

A back up heater is often installed in the furnace.