biggest hurdle to the likelyhood of achieving a great symphony grade is the user (both technical deficiencies and taste deficiencies are common).  If you work in a main market area and are capable of producing wonderful consistent grades (both subtle and heavy ) you probably wouldn't  use symphony.

Why is this? TCurren has covered many reasons but I would add a few:

It is not a grading machine. I think this is a mistake ,and should be rectified by avid, but I am in the minority here.

There is no easy way to spatially define corrections. It is all very well picking a hue that covers an area that need a subtle lightening but if you can't loosely fence it off then you'll probably end up with an unwanted correction somewhere else as well. Of course you can work around this limitaion with patience and skill but this should not be needed.

the interface is poor and fiddly

Beyond the 444 limitation (lets hope that changes) there is also a problem getting high quality material into the system.  DPX don't come in well (hopefully this will change with the next metafuze release) nor is 10bit material ingestible apart from tiff16 or tape.

regards Michael

ps Don't blame the guy you are using I'm sure he is good at many other things. Grading at the highest level is a job in itself.