In this line secondary is counted as a replSet member only if it has better score than the previous one.
https://github.com/Percona-Lab/mongodb_consistent_backup/blob/master/MongoBackup/Replset.py#L113

If there is more than three members in replSet it will randomly raise "Not enough secondaries in replset %s to safely take backup!"

I don't know if it's intended for secondaries with too big lag to be counted to QUORUM(https://github.com/Percona-Lab/mongodb_consistent_backup/blob/master/MongoBackup/Replset.py#L119) or not (https://github.com/Percona-Lab/mongodb_consistent_backup/blob/master/MongoBackup/Replset.py#L109)