I have never understood how some vendors use 300V insulated round cable in MCC wireways without running headfirst into NEC Article 725, but that's between them and their customers.
The knurled connectors in particular are alarming to me: a conductive, sharp, abrasive item in a wireway with unsecured wires carrying high voltage, AND it's the first thing you're going to grab.
Rockwell Automation, as you've probably seen, uses the flat cable extensively. Having the wires parallel to one another gives you noise cancelling that is almost as good as shielding. Placing the flat cable behind the wireway also makes it nearly impossible to damage.
The proximity of the MCC bus is less of a problem (at least in the MCCs I've used) because the bus generally carries nice clean sine waves, while the output of a VFD (which is going to be lower power but much closer to the droplines) has all kinds of harmonic and noise components.
In nine years of commissioning and troubleshooting MCCs with flat-cable trunks, I have seen only one noise-related problem, and that was with an unapproved power supply and untwisted power wires that ran 20 feet through the wireway past the output of Yaskawa drives.