Errm, let me parse that 
- PureOS on non-Purism hardware: Yes, no microcode updates.
- Purism hardware and other OSes: Well, as far as I understood the microcode update mechanism, this is a one way street, i.e. an OS microcode update mechanism will only upgrade not downgrade. So you will not create a conflict but the OS may have a newer one and upload this or Coreboot may have a newer one and then the OS does nothing.
That the BIOS, in this case Coreboot, installing some version of the microcode is pretty normal, so this case must be handled gracefully anyway. But the case for PureOS on non Purism hardware is a bit annoying indeed since there will be no microcode upgrades unless the BIOS that’s running on it gets upgraded.
Cheers
nicole