I pointed out above.
Dark Ranger Anya and Velonara's Black Arrow curses enemies. This is the same ability that Sylvanas had as Warchief, and it was an actual Curse type ability. Banshees are known for using Curses, established since Warcraft 3 and carried over into WoW.
The Hunter's Black Arrow involves no actual spellcasting or curse involved, it is treated as an enchanted arrow that applies a just a magic-type debuff. There is no curse or Banshee ability involved.
On top of all this, Dark Rangers tend to have unique abilities not found on Hunters, such as use of Shadowburn Shot, Smoke Bombs and even Wailing Arrow.
https://www.wowhead.com/spell=205421/wailing-arrow
"
A deafening banshee's wail erupts from the arrow's impact, inflicting Shadow damage and silencing nearby enemies."
Used By:
Dark Ranger Alina
Dark Ranger Clea
Forsaken Dark Ranger
As for Sylvanas' situation... we don't actually know whether she is the only Banshee inhibiting a body, and whether or not that applies to other Dark Rangers. It's left ambiguous, just like asking whether any Demon Hunter can actually change into a Demon Form like Illidan and not just the shadow Illidan form which we know any Warlock can do. There was no other Demon Hunter with visible Horns or Wings, or any one that would use a unique form that looks like Illidan. Every time we see any NPC it was always using the same Shadow Illidan model that Warlocks used.
It's a correlation argument. We see Illidan as the only one who can take up a non-shadow demon form, so we assumed no other Demon Hunter could look the same way. Yet the truth is it's always been ambiguous and ill-explained, until retroactively confirmed by the addition of an actual Demon Hunter class.
The Dark Ranger NPCs containing 'Banshee spirits' within them might not be addressed unless we actually have a playable Dark Ranger class. That's the reality of the situation, and we can't just bank on Dark Rangers being 'different from Sylvanas' when we don't actually know the true origin of a Dark Ranger and their ties to Sylvanas. Look how Delaryn Summermoon was completely against Sylvanas in life, but became her servant in death. There are still things left unexplained, and Blizzard hasn't indicated they are willing or ready to explain any of those relationships right now.