The original poster was asking about connecting EFI and MSD. The FAST EFI instructions are very specific about connecting to the battery.

I think it would be OK connected to some other battery voltage source if the wire size is adequate with good connections and no large current changes on the line.

Much of what has been discussed is a matter of degree. Connecting loads like lights and fans off the alternator works fine because it is not a problem if there is noise on the power line to those devices. The possible down side is there could be a slight change in the regulated voltage depending on where the voltage regulator sense line is connected (internal regulated, regulated sensing battery voltage, or somewhere in between?), again the effects would likely be too small to make a difference in our application unless there is fairly large resistances, like 0.10 Ohm, in the wires and connections. For example, alternator charging battery at 10-Amps, that 0.10 Ohm is a 1.0 Volt drop between the alternator and Battery.

The circuits in the EFI are much more sensitive to noise. The voltage inputs are fed to analog-to-digital converters that are converting voltages at a high frequency and feeding the information to the processor which calculates ignition timing and injector pulse timing, and then controls the power to the injectors, ignition coil and other output devices. I'm sure the manufacturers design in quite a bit of filtering, but I'm sure many problems called into support were because of connecting the EFI unit to power incorrectly.