what's "wrong" with diesel generator power?

It sounds like the voltage and frequency of the temporary generator power were both overly high.

• Grid-tied solar power systems measure the frequency and voltage of incoming power and do not supply their own if it is not within expected ranges. This is for a number of reasons that can be generally described as "if there is a problem, don't make it worse", including but not limited to;
  • If there is a power failure, it is not desirable for home solar power to try to run all the neighborhood loads, because it will almost certainly be insufficient capacity and unable to provide stable power — and even if they could, there would be a phase synchronization problem upon attempting to restore power.
  • High frequency is a sign of momentarily under-loaded generators (just like a motor runs faster without load, so does an AC generator), as is high voltage, and adding more power to the grid in that situation would be bad. (Note that moderate frequency shifts are actually normal, as they effectively act as a way to signal between multiple power plants to help them share the load; the way this works is partly simple physics and partly deliberate regulation and is a bit more than I'm equipped to correctly explain in a brief paragraph.)
• The hums from the magnetron, fan motor, and turntable motor in your microwave will all be the line frequency and multiples (harmonics) of it.
• The igniter has to create an even higher voltage from line voltage, and such a circuit might fire faster if it gets more input voltage.

There is no inherent reason why a portable generator should have consistently high (rather than low or just right) voltage and frequency — they're just not necessarily as rigorously controlled as the many interconnected grid power plants.

The standby mains voltage and frequency would not be as stable as the grid.Where would exported solar energy go if the neighbours were not drawing much power ?.The voltage waveform would probably have more harmonics present .This is why the power is not as good .Also the power is expensive to generate .