Each device draws different currents which due to the non zero resistance of the traces will cause the voltage on VCC to sag and the voltage on GND to rise above zero when any device is drawing current, this VCC sag and ground raising will change what VCC and GND chips further down the line see, and it will fluctuate with load so you'll get crosstalk between devices. Decoupling caps will help reduce noise but it won't change the fact that your ground will actually be above ground relative to other parts of the circuit.