A Wraithlord is hit normally, but wounded on a 6 with heavy bolter, 5+ with starcannon, and gets a 3+ save against the bolter. The Harlequin one gets a 4+ coversave against starcannon-fire. And if one of them get close enough you are in trouble.
I would dread facing 6 of them, just as six Marine dreads would be a handful indeed.