Are frost fans safe?

Big question.

They consist of a large mass of metal spinning at high speed, so there is an inherent risk if there is a structural failure.
This risk will be affected by the quality of the components, design, installation, maintenance and hours of operation of the frost fan.
However, airplanes fly every day with propellers managing to stay attached to their engines and helicopters (which are probably the closest analog to a frost fan) are pretty reliable pieces of machinery.
Of course the aviation industry has enormously stringent safety regulations which frost fans are not obliged to follow.
Recognising that there is a risk associated with operation of a frost fan the Department of Labour has released an 'Accident Alert' in February 2006 (http://www.osh.govt.nz/publications/series/aa09-windmachines-safeoperation.html). This is designed to provide a guidance for safe operation of frost fans. Their guidelines are as follows;

• Thoroughly read and understand information provided in the wind machine operator’s manual. Pay particular attention to descriptions of safety procedures.
• Before using the machine each frost season, always inspect for damage or disrepair and make sure that all drive train covers are securely in place. Always check gearbox oil levels prior to use and lubricate according to the manufacturer’s instructions.
• If a wind machine fails the pre-use inspection, remove the equipment from service until repairs have been carried out using lockout procedures.
• Keep vineyard sprays from hitting wind machine engines and towers.
• Shield wind machine engines from irrigation sprinklers. Operating precautions
• Never operate a wind machine when winds are at 7 km/h or greater.
• Always stand outside of the plane of rotation of the fan blades. When near the wind machine, keep the wind machine tower between yourself and the fan shaft.
• Never operate a wind machine during foggy conditions.
• Only authorised and suitably trained people should climb wind machine towers. Where practicable, use an elevating work platform.
• Wind machine repairs and seasonal maintenance shall be performed by the vendor or vendor-approved contractor.
• Never run the wind machine engine for more than one minute with the clutch not engaged.
• Always run the engine throttle at fast idle for five minutes before operating the wind machine at full speed.
• Wind machine shut-down procedures include returning the engine throttle to fast idle for two to three minutes followed by reducing the engine speed to idle, disengaging the clutch, and shutting the engine off. After the fan blades have stopped turning, re-engage the clutch.
• Never allow the fuel tank to run out of fuel when a wind machine is operating.
• Do not make adjustments, alterations or modifications to the machine without first consulting the supplier.

There is a lot of good advice there, but I do not believe that it is binding on operators.