It's generally not a good idea to rely on the chassis as the return conductor.
Magnetically coupled interference happens because a stray magnetic field is coupled into a magnetic loop in your circuit. The smaller the cross-sectional area of your magnetic loops, the smaller the interference.
Consider twisted pair ethernet. This has little susceptibility to interference because the twists mean the area between the conductors is small.
If you use the chassis, your currents are travelling around a loop that is probably several centimetres across, and so gives a wide area to receive interference.
Additionally, you may not have a good low resistance connection depending on factors like corrosion.
What about cars? Cars use the chassis as ground. But they're full of high current components (lights, pumps, etc) that aren't susceptible to such interference - when connecting to a device that is sensitive, there will be a ground wire on the connector.