Can't say I'm a fan of Sloane but I get that some people prefer her over Reyes. I tend to pick Reyes but to me it's really a choice between to evils if you will.
I don't really consider her a killable character though. I don't get to kill her. I only have an option to save her. So she maybe saveable not killable in my book, if you can appreciate the difference.
I just personally dislike Sloane because she's got a couple of traits I just really don't like more than the bad traits of Reyes.
The interesting thing is that you need to help Reyes as part of the the path to getting Kadara settled. So in essence you build a relationship and then suddenly you're supposed to choose between the two?
It would've made more sense if you had a choice to help one or the other. So yeah, I do get it when people feel BW prefers people to choose Reyes.