Thought I could chime in after some testing of my own setup.
Made my own USB/BT expansion board for my own ECU design. Dealing with RS232 adapters sucks living in 2016...
Been testing this setup on multiple vehicles now, both with expensive shielded cables (without ferrite beads though), and non-shielded cables straight from eBay/China.
Have had no issues in normal operation at all with neither of them.
I did manage to torture the thing to the point of comms loss on the test bench, physically wrapping the USB cable around the GSXR pencil coils while reving at high RPMs using an RPM stim, lol.
What I can say for certain is that resistor plugs are considered, at least for my part, as mandatory in a car running EFi and especially with USB comms because of the way lower thresholds when distinguishing between hi/low levels between the datalines compared to RS232.
One would believe USB to be almost infinitely robust because it uses a common mode signal, but it's not that way in the real world unfortunately.
The attached picture is of the rev.B, while the schematic is for the rev.C after I implemented some additional filtering on the power and datalines running from the PC. The rev.B have proven to be stable, but i felt that rev.C was of better worst case design practice
The Pic10F200 is just a USB watchdog monitoring the RX pin transmitting data from the ECU MCU. It will reset the FTDI chipset after eventual comms loss, thus enabling the USB link to get re-established without the need for disconnecting/connecting the USB cable. It has only proven useful when torturing in the test bench
The Bluetooth link will also work with the USB cable connected, and no hardware input is needed to switch between cable and wireless comms.
Ahh, and yes, it's Tuner Studio and not my own firmware... Fred, you know my shit...