...it's plain and simple, better than every other aspect.. it also makes warp spiders, fire dragons, swooping hawks, and scorpions redundant. no, wait. the shadow specters are in and of themselves redundant. you know what sucks about this? the models look kickass. welp, RETCON TIME!
Spidies went down to #2 due to that I found out about the SS As a german it's even cooler to call the Shadow Spectres 'SS' xDDD
Pretty much, did you read about Ghostlight? If multiple SS activate their ghostlight they can shoot across THE ENTIRE battlefield .... and hit precisely. Mainly their weaponry is AV, therefor imagine shooting a Leman Russ on ~10 miles distance XD
I don't think they overlap with other classes, they may be op because of high mobility and sniper-antivehicle weapons... well... they will definitely be op as fuck