She was an extremely-advanced martial-arts practitioner; many of them are startlingly-competent.
She had limits, Nic wiped the curb with her.. Then she removes her own casts, one minute she can barely walk, the next she manages to surprise and take down a Valkyrie?
I personally knew a "Murphy"-like person -- cute, small, blonde, senior aikidoka. Nice lady, and when she came at you seriously she was the scariest person I've ever met (this with years of karate behind me). I have literally seen another black-belt take a glance at her then turn and run away, on the mat!
Maybe when she was physically whole and in shape, but as of Peace Talks and Battle Ground Murphy wasn't. Even if she were in splints and doing physical therapy all along after Nic got done with her... She apparently wasn't because she had to cut her cast off, if you have ever worn a cast for a time or recovered in splints after orthopedic surgery, you know the limb doesn't instantly recover, even if you are doing physical therapy all along..
When Murphy added the Sword at Chichén Itzá... yes, that turned her dial up to "11," and I liked it -- for her character, for the story -- that she decided to put the Sword back down again, afterwards. The post-Changes Murphy was back to being a mortal, not Supermurphy; but she was much more being stretched to her limits, and barely-keeping-up. Before that, with a couple of exceptions (vs the Nightmare & vs. Papa Raith), Murphy was mostly on top of even the supernatural situations.
Which just wasn't realistic even for a fantasy.
From the high-fantasy action-adventure aspect of the Casefiles, Murphy was no longer a "viable" adventuring character to accompany Harry.
Which Murphy should never have been to begin with in my opinion. When we first meet her, smart, strong,a good shot and martial artist, she didn't need to become an action-adventure figure to be compelling. Michael doesn't need any super human abilities, he had a Holy Sword, but he does pretty good without that.
Actually when you think of it, Murphy's actions was really a form of suicide. With her injuries she wasn't up to really fighting any pitched physical battles, and actually could have made things worse had Harry been distracted trying to protect her. Yes, she caught a bullet from a paranoid cop and died, and did manage to take out a giant with a bazooka before she died, but in the later physical battles where even the likes of Odin and Mab barely made it, she wouldn't have survived. Yes, hard to stay behind at Mac's place to protect the people there, but Harry wasn't being over protective of her in doing that. Actually it was her ego once again that she knew better, so she failed to follow orders, that's what got her killed.