It's not a question of skills or talents but a matter of expectations and roles. Luke,Kate,Bertinelli were and are periphery Bat characters,though it might change for the former 2 eventually it's still how they started and how they are. Time and development will facilitate integration in to the core family should it happen eventually but at the same time there is no guarantee of that either. Duke and Harper are forcing their way in to the core family even though there is no place for them in terms of what they offer story wise or skill wise, no one would have any problem if they existed as side characters. Imo they dont even fit in the periphery family,not with Luke,Kate,Azrael,Bertinelli who between them cover any need for characters like Harper and Duke.
Secondly you have to take in to account the legacy and stories that Dick,Tim,Jason and Damian bring to the table,conversely Luke only had to compete against David,Bertinelli against Wayne and Kate against Kathy. There is a huge qualitative and quantitative difference here,it wasn't particularly hard for either Luke or Kate to show how different(and superior) they were from their predecessors,Duke and Harper are inferior characters retreading the same territory pretending they are somehow on par with those who actually died on the job.