This is simply a matter of opinion.
I don't think Stephanie or Cassandra as Batgirl run are overrated, since I enjoy both a lot more than Barbara as Batgirl run (I like Batgirl Year One, but I don't find something incredible either).
However, there are other people who will think that Barbara as Batgirl is better or has a similar level than the other Batgirl run, so it isn't weird they think Cassandra or Stephanie run are overrated.