I had to do quite a bit of reading on this subject, but for a particular installation only, rather than for general use. So I am not remotely qualified as an expert but feel that I have a reasonably informed opinion to offer. My application was a non-NEC compliant installation for temporary power on an experimental aircraft system.:
Ungrounded systems mean that a ground fault won't shut your system down. If you get a _solid_ ground fault, then a small amount of current will flow, and you will be able to continue operation of the system. Your ground fault detection system will tell you of the fault, and you can perform an orderly shutdown, locate the fault, and repair it.
The problem with ungrounded systems is that you don't have a solid voltage reference. Because of the capacitive coupling between the system and ground, if you have an _intermittent_ ground fault, you can actually pump the voltage between the system and ground to levels that substantially _exceed_ the line voltage of the system. This can lead to a situation where a minor fault on one piece of equipment can lead to high voltages and insulation breakdown all over the place.
It is my opinion that a good ground reference and proper breaker coordination are vastly preferable to an ungrounded system.
If you actually require the continuity of service benefits of an ungrounded system, than I would suggest looking at 'high impedance grounded' systems. These are wye systems where the neutral to ground bond is made using a resistor (50-100 ohms on a 480/277V system) that provides a good ground reference but limits ground fault current flow to a low value. Depending upon how the system is designed, the current through a ground fault will be perhaps 2-10 amps, which most circuits will tolerate indefinitely. You need to use the same sort of ground fault detection used for ungrounded systems, but in the event of an intermittent fault you won't see excessive voltages. Even though this is a wye system, under the NEC an impedance grounded system cannot supply line-neutral loads, and is thus limited to the same sort of loads seen on a delta system.
The companies that sell neutral grounding resistors will have a lot to say about the dangers of ungrounded systems and the benefits of resistance grounding