One of the main benefits of a drainback system is that there is no need for a heat dump. When the heat store reaches its high limit setpoint, the pump shuts off, and the collector drains back. You would think that there would be a problem if the system started back up when the collector was super hot after being drained, but the thermal mass of the collector is relatively small compared to the amount of heat it would take to immediately boil the incoming water. Unless the collector is super huge, it stands no chance of raising the water from 150 degF or so to boiling in one pass unless is is cherry red hot. Sizing the collector is actually easier for a drainback system due to this shutdown at max thermal input from the sun.
Recirculating systems are another story. They are designed to always have liquid in them. On those hot, sunny days, you have to have a heat dump active to prevent the brine solution from boiling. Just a few degrees of delta T adds up on multiple passes in a hurry when there is no load. All recirculating systems are required to have a blowoff valve in them (in America) to prevent a catastrophe in case the power goes out. Many have redundant backup pumps that only operate when the power is out. Due to the heat dump and its fan, recirculating systems are less efficient than drainback systems. The extra hardware (heat dump exchanger and plumbing, assorted valves, sensors, controls) adds a lot of expense to the system as a whole.
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