In general, there are many ways for DC-DC conversion: SMPS, charge pump, linear regulators, zenner diodes, even resistive dividers. But lets say I want to create 2 voltage rails, +Vcc and -Vcc.
Most of us would exclude linear regulators, Zener diodes and resistive dividers from a list of DC-DC converters and call them regulators as they burn-off excess voltage rather than convert it.
I can use an explicit negative voltage converter like an inverting charge pump or a buck-boost inverting topology converter, or even build a dual PSU that produces 2 rails centered around earth.
You don't have to use earth. You can call any point on your circuit GROUND and use that as your reference for everything else. A battery-powered, hand-held device, for example may need a dual supply but can't have an earth connection.
Can I achieve the same by just have 2 DC-DC converters, calling the middle node GND, and calling the low voltage -Vcc and the high voltage +Vcc?
Yes. You might be able to avoid one of them if your battery voltage or primary power-supply suits, say, the positive rail. Then you just need one DC-DC converter to generate the negative rail.
Are there any downsides to this method rather than using an already established GND and pulling a rail down with reference to that, aside from potential shorting risks to earth?
No, if the outputs are isolated from each other you are free to connect any one point on one to one point on the other. That's standard practice and good engineering.