The danger of having a 3 wire feeder with a shared EGC and neutral is that you end up with voltage on your ground. Before the exception was removed for separate building feeders the rationalization was that you were providing a local ground electrode system, much like what happens with the service. The fact is, if you actually look at the GEC in your house, you will usually see neutral current there too and that is also demonstrating a voltage drop in that 3 wire SE cable neutral/ground.
The problem inside your house is that you will see a voltage gradient between the grounded case of equipment from this 3 wire sub and a truly grounded item like a cold water pipe in a metal plumbing system.
