Unsullied are trained for formation assaults, not for policing alleyways. Even still, the 7 of them took out a good 30 harpies. The promo for next week makes it look like Selmy's dead, but if they were going to do that they might as well have done the throat slit instead of having Grey Worm save him. Still, they could easily combine Selmy and Jorah's plot going forward.