The SCR shunt type consumes more energy in the Regulator itself than the bike is using and dumps a ton of current into the heatsink (feel yours & just see how hot those things run!!!! - don't touch it - you'll burn yourself - seriously!) The problem is exacerbated because their efficiency goes even lower when they get HOT so it's a vicious circle. Heat is the number 1 killer of these devices.
Incidentally its a misconception that shunt type work harder with increased load i.e. higher-wattage lights, heated vests etc - actually, the higher the load on the output, the less work the shunt regulator does in dumping that excess energy and will actually run cooler!!
The FET has extremely low resistance in conducting state and this results in a lower dissipated power from the device while conducting load current, as opposed to the SCR which shunts the maximum current across a significant volt drop, resulting in a higher dissipated power - and resulting temperature, much more so than the FET device.
