I use an Arduino Uno (not a clone) and a gShield v5 with my SO2, and I have never personally experienced a USB disconnect when running gcode files. I have tested all of the popular Grbl releases with my setup, and currently alternate between LarryM’s 1.0c Grbl and the latest official Grbl v1.1f. I also use four Arduino Unos, each with a different Grbl version, on my software development computer, and never experienced a USB disconnect with them either (excepting those that that I stupidly caused 
).
It is my personal belief that many of the cases reported of USB disconnects can usually be traced to one or more of the following:
1 - running the spindle ac power cords in the drag chain with a machine’s signal cables.
2 - poor, or missing, electrical noise suppression circuitry/techniques.
3 - the control computer’s settings.
4 - running other computer programs while also sending gcode to a machine.
5 - inexperienced users making changes to Grbl settings.
Unfortunately, software bugs do exist in all of the programs we use (including Grbl), and I, and other program developers, work long hours to eliminate them when discovered. I am personally embarrassed whenever a serious bug is discovered in one of my programs that causes a bad carve/engraving for my users, and I set aside my own hobby craft work until I can find a solution and issue an update.
The reports/complaints we receive about problems are often lacking in important details we need to investigate/duplicate the problems, and provide solutions to them. The help from experienced users like Jeff Woodcock, Larry Mayberry, Larry Gonshak, to name but just a few, are greatly appreciated. Without them my work would be much harder. My sincere thanks to you all, named and unnamed.
As to Grbl itself, Sonny Jeon, the chief programmer/developer, deserves far more recognition and reward for gifting all of us his time and talents as evidenced in Grbl.
John