That's reasonable. The list of volumes (local dirs) is explicitly listed so the config isn't portable even when specified as a percent, but it's one less config that isn't portable.
IIRC Koji's perspective was that an admin doesn't want to specify the count or percent of valid volumes, but that after a set number of failures the host should be considered faulty. Eg if it's lost two disks there's probably something wrong whether the host has 6 or 12 disks, ie assumes disk failures w/in a host are correlated.
Ideally I think we should collect data (eg an X core host can still function well with Y% disks) and not require users configure this at all - it would be enabled by default and the daemons would take themselves offline when they've determined they don't have sufficient resources.