Not necessarily, but maybe. The problem is that as soon as you exceed the power ability from the USB port, one or more drives will get powered off, which means that if that drive is in the process of writing something, that write process will fail. IF that write is to the directory, you could lose the entire contents.
Again, the issue is that the total power from the USB port should not exceed the recommended power for the port. IN the case of USB3, if the MBP port is, in fact, USB 3 then the spec for the port. Here is what Wikipedia says about the power from USB3 ports:
Power and charging[edit]
As with earlier versions of USB, USB 3.0 provides power at 5 volts nominal. The available current for low-power (one unit load) SuperSpeed devices is 150 mA, an increase from the 100 mA defined in USB 2.0. For high-power SuperSpeed devices, the limit is six unit loads or 900 mA (4.5 watts), almost twice USB 2.0's 500 mA.[9]:section 9.2.5.1 Power Budgeting
USB 3.0 ports may also implement other USB specifications for increased power, including the USB Battery Charging Specification for up to 1.5 A or 7.5 watts, or the USB Power Delivery specification for up to 100 watts.[11]
You can look at the tech specs for your Mac at Apple to see if they define which, if any, of these standards apply. But in general, I would not assume more than the defined 900mA unless I knew with absolute certainty that it could handle more.
Once you know the total capacity of the USB port, you can add the demands from the total set of devices to see if they can all draw at the same time. If not, don't do it.
