Quite simply, there is zero evidence on if Shimrra or Rhea could take one another in a fight as the only fights they've had are against the same guy in...radically different circumstances.
We should NOT use Rhea's fight with Luke as a benchmark for him. As pointed out above, Luke does not begin the fight at his best and though he's recovering as it goes, there is no indication he's at his best or near it when he engages Rhea (Who is a member of the Sith Council of Lords and probably not a weakling)
Then he has to take on her and her apprentice at once and gets injured there in the process, leaving his body still tired and wounded at which point he kills her.
Even at Shimrra, Luke was noted to be exhausted from all the fighting he'd done. ...the times we have seen a full on, totally fresh Luke fight? Not many. That contributes.
Luke isn't infallible, he can be challenged and by some people he might even be beaten. But he is completely top tier and has shown so.