I have MacBook Pro Retina, 15-inch, Mid 2014 and use Traktor PRO 2.8 and Traktor 3 both, I use X1 MK2 and used to use Sound card of DJM 900 Nexus/DJM 2000 Nexus or any mixer provided by club having inbuilt sound card mixer.
Problems can arise when your hardware device is too old or not supported any longer. DJM-900 SRT mixer after connection of vdj8 on what PC screen. NOTE, Pioneer DJM-900nexus Mixer, USB cables. The unit features a 24-bit / 96 kHz professional-quality DAC processor, the Pro DJ Link system, wireless functionality, and a full color LCD display.