Downsides of Sara coming back? Well, I thought the gravitas following Sara's death was well done, and the emotion there is undercut if it turns into another fakeout.
Plus the Sara's dead, Sara's alive, Sara's dead stuff is played out at this point. And that's going to be super awkward, because Quentin has been so obnoxious lately, and now it's over a daughter who isn't even dead anymore...again. There's really no version of it I can accept that doesn't involve Oliver (or Thea or Diggle, if he's unavailable) backhanding Quentin across the face. Just really slap the **** out of him and tell him to get the **** out of Starling City. I just don't see anything else they can do with his character after this that I'd have even the slightest interest in seeing. If Sara were still dead, I could see a redemption plotline, but with her alive his story arc just seems too dumb.
I mean, I didn't want Sara to die, and I wanted her back after she did die, but here's how it looks for me at this point. Sara dies (for real this time, honest), everyone's sad/guilty/angry, her weird sister tries to be her, Nyssa didn't know the Lazarus Pit brings people back to life, now there's free do-overs, Sara is alive again and the showrunners weren't really serious about the whole dying thing, she changes the way she dresses because her weird sister has stolen her identity (I'm going to go ahead and say that maybe Laurel should change the way she dresses), then she has to go back to her home planet so Laurel can continue being her in her place, even though she's still here to be herself. Did I mention how awkward the Sara and Laurel reunion will be, too?
Also, yay more Sara on this new show! That's not sarcasm, I'm definitely glad to see more of her, but the parallel timelines option was the more elegant solution in my opinion.