[AgentEpsilon]

Personally, I use rEFInd to manage my MBP's OS installs. I know that it installs itself to the EFI partition, but would that cause this service to identify an insecure/corrupted/whatever EFI?

[kogir]

rEFInd is just an EFI application. It doesn't replace the system EFI firmware (which runs before, launches, and provides services to rEFInd).

As to whether or not this tool will flag it, I suppose you'll find out shorty after installing High Sierra.