It is a problem on the A320 that as pilots we are told about. For example, you take off, you lose pressure in the yellow hydraulic sytem, the power transfer unit (in this case driven by the green system) attempts to pressurise the yellow system. If the yellow system was lost due to loss of fluid and not a simple pump failure the PTU will be attempting the impossible and if you are not fairly quick to turn off the PTU on the overhead panel you can overheat the other system that is driving it.
During takeoff you will have many things to do and in the drills it takes a while to get to turning off the PTU so hence this can cause overheat. We were issued an aircrew notice reminding us to kill the PTU if we have a loss of fluid.
Either way the aircraft is fine to fly on one hydraulic system, just not ideal.