Voltage drop when powering microcontroller from 5v

This is intended to be so. The 5V at the master module is for patching, there is a small resistor behind it so you cannot damage anything when you short it to ground. You wouldnt notice when you patch it to a CV input for example, because that cv input is high impedance. However, driving a MCU or another IC's power rails, is something else. In this case the resistor behind it will act as a voltage divider together with the IC's internal resistance and the voltage will drop.

The brAEdboard modules +5V and Gnd are truely ground, this means there is no safety resistor in there. So its perfect to power other microcontrollers with it.

Another way is to directly use the pins from the bus connectors to power your microcontroller.