I'm no expert, but it seems to me that gang tooling could be difficult to set up with adequate clearance if you need many tools loaded at once and are turning 2-3-in OD work. On the standard gang tool block, the tool-to-tool distance is around 2" so interference between adjacent tools is possible with work of 2" OD or greater. That wouldn't be a problem with the QC system but then you have to make all or most of your tool changes manually.
On the other hand tool changes with the turret are slower than gang tool changes and my suspicion is that the repeatability of the gang and quick change tooling (especially gang) is a bit better than the turret.