Personally, (and it's only my opinion) despite owning a bit a FEC & Nightaunt (from when they where part of the
VC) i don't use them with
SBG.
Since your battle trait and faction rules don't work on them and you straight up get "nerfed" units since they don't get theirs either. There's no point to bring them. from a competitive standpoint
Especially when in addition the
SBG book has a really really good roster of units ans there isnt really any unit in FEC or NH that don't have an equivalent in
SBG if not straight up better in
SBG.
Buuuut... in a friendly game, sure bring models that you like and have fun with them
BTW, Varghulf and black coach should defenitively be
SBG unit (and mortis a NH one)
Edit : On the other hand, OSB have some units that would be really nice (crawler for exemple) but unfortunately we can't allies them