When using the unicast-switch ("-u") within a simple two machines master-slave-configuration no sync can be accomplished (at least for me).
Additionally according to George Neville-Neil no pure unicast communication is possible in IEEE-1588. Nevertheless the help message of ptpd2 ("-u ADDRESS Unicast mode: send all messages in unicast to ADDRESS") suggests such a mode.
Proposal: At least clarify the help message, i.e. that only a subset of the messages will be sent by unicast instead of multicast.