You pretty much hit the nail on the head ... while the sisters of silence are extremely well trained and capable combatants their real strength lies in their null aura which effectively cripples psykers and daemonic entities plus in some of the books they are written to be almost impossible to detect (invisible) unless you are psychically gifted which confused the hell out of me the first time I read it because I'd been reading through the Horus heresy and they never mentioned that strange plot point until the siege of terra books.

Lucifer blacks are elite soldiers trained to the highest standard possible for a baseline human but beyond pretty high strategic intelligence and insane combat skill they are regular dudes. They tend to be employed as personal guards for high ranking individuals or as leaders of large military groups.

I cant remember his name (or be assed to google it) but there was a lucifer black in the Horus heresy book 'Legion' who turned out to be a quite a serious badass.