Which also seems to be a hallmark of the show Unsullied. They're pretty big on keeping their backs to their enemy at all times. Oh, and we saw several more of them scream in pain in this last episode as well. There's really no reason to believe, based on what we've seen this season, that Dany's army of Unsullied could have any role to play in conquering Westeros at any point.
There's a reason why the Harpies in the books would creatively lure one Shavepate/SS/Unsullied at a time into a bad situation and kill them- it's because openly engaging them at any point would have been ludicrous.
What have we seen the Unsullied do in the books? The defeated a numerically inferior force in the books which basically broke at the outset of the battle, and then took Mereen through mostly subterfuge. They were equally incapable of keeping the peace and quelling the SoftH in the books as well as the show.
Also, remember that these guys are being paid a wage now and have leisure time. They aren't the robotic death cult which held the gates of Qohor against the Dothraki horde anymore. We'll see how they perform at the battle of Mereen in the books, but I would guess Dany's army of Unsullied isn't as invincible as you think.