Cass, actually, was kept away from metahumans. She had very little experience with them, particularly in hand-to-hand, and Bruce definitely didn't let her near anyone remotely on Ivy or Clayface's level if he could help it. And this was early in her career, well before the one time she fought Ivy (which job she needed Bruce's backup to finish anyway).
I don't think they come off incompetent here at all. Harley doesn't just have the edge psychologically here, she's got a whole knife drawer. She's spitting on their *fallen.* None of them are calm or thinking straight, and even with that, she still needs backup from an entire barfull of thugs to make her escape... after which they immediately find her, bag her and hand her over to the authorities. Win for the Bat-team. They're not the ones who let her get away.