Primary side sensing on dc-dc converters has the great advantage that it eliminates the need for feedback components, particularly opto-isolators, in low-power mains power supplies - for phone charging and LED driving for example.
Algorithms and topologies have developed to the point that 5% output current and voltage regulation is possible.
However, amongst the disadvantages of primary-side sensing is that sensing, and therefore regulation, is only possible when the conversion transformer is being driven.
Source:
http://www.electronicsweekly.com/Articles/2013/04/06/55891/iwatt-brings-back-opto-isolators-to-tackle-output-droop.htm
