So, a few thoughts on Calia, but first, Voss…
Voss is a character they’ve built up slowly from Cata. The Cata Forsaken starter quest zone has you running into her as she comes to terms with being raised as a Forsaken and the fact that the Scarlets now want to kill her. She never actually joins the faction though. We meet her later in MoP, where she’s now dedicated to eradicating necromancy, and has us help her finish off the Scarlets and later Gandling in Scholomance. We find her continuing this in WoD from an Inn / Tavern Daily quest. In Legion, she’s now a Rogue Order Hall follower, but still neutral. Only in BfA is she actually now a member of the Forsaken, and even this isn’t ever really elaborated on much. So, the two takeaways here are that she was built up slowly over the course of expansions, and her being a prominent Forsaken is a relatively new thing.
Calia follows quite a different path. Calia was created mid to late Wrath in a book - far too late to be added to the playable expansion. She wasn’t mentioned again until Legion, when she’s a Priest Order Hall follower. Even here, she doesn’t really have much of an impact on the OH story (though to be fair the Priest and Paladin OH stories were the same anyway). Her story picks up in a book, where she’s killed and then resurrected by Anduin and Faol channeling a Naaru’s power into her. She willingly chooses to become undead and proceeds to stay in either the Priest OH or Stormwind for the remaining time. Her next appearances are Alliance quests, with her final appearance with Voss being to work with the Nelf Forsaken (but not the traditional Forsaken). Notably, the Horde NPC can’t actually interact with her for any of this.
So, that’s the first issue. Voss has her story built up in-game over the course of several years / expansions whereas Calia is basically poofed into existence in BfA, only appears in a few questlines (most being Alliance), and never even really interacts with the faction proper. It takes time and exposure to get people to like a character, and Calia clearly has neither.
Now, as for the second issue, she’s not only not really a Forsaken, but almost the antithesis of one. My character is a Vanilla era Forsaken, so it was originally killed and raised by the Scourge’s necromancy (or at least this is heavily implied). It probably also had to do some things while in service to the Scourge, or at least had some pretty terrible things happen to it. It’s also implied that being raised by the Scourge does cause some negative side-effects. DK’s, for example, must constantly inflict pain on other things or else they themselves suffer. Either way, it spent most of its time lvling up dealing with the affairs of Northern Lordaeron. Later on, it would go to Northrend to confront the Lich King. It’s since done other things on behalf of the Forsaken and Lordaeron (though admittedly there’s not much else the Forsaken canonically do I guess?). Oh, and as for the name Forsaken, it’s mostly because the other races and groups initially spurned them until the Tauren agreed to welcome them into the Horde.
Calia, on the other hand, never went through any of this. She fled to safety and abandoned both the region and her identity. All things considered, she did the one thing many in the Forsaken wished they could have done, but didn’t - she lived. When she was finally killed pre-BfA, Calia was raised by the light, so she suffers no ill-effects from this. Additionally, she specifically chose to become undead when she could have returned as a human. So, in other words, she completely abandoned (forsook) the entire region when it suited her, and only returned and became an undead because she now feels it suits her interest. Hardly a relatable story for the average Forsaken.
More fundamentally however, it feels like another attempt by Bliz to try and revise and rewrite their own lore and history. Why do the Forsaken suddenly need a light based undead to come in and “fix” the faction? Why can’t someone from within the ranks - or at least heavily tied to the questing experience (either former SC or AD members) be the one to do it? Also, why does it specifically have to be a new Alliance character tied directly to Anduin? We already have him as bff’s with Baine, and now he somehow features into the upcoming book with Talanji and Zappyboi. Why do we need Anduin affiliated characters all over the Horde faction? As with a lot of things with the Horde, it feels like them just trashing the original characters and narratives for their “new and improved” Horde. (Though this can also be said for the Alliance too. Never forget that, back in Vanilla, Magni was a pretty terrible father to his daughter.)
The faction’s been around for years, and we’ve had plenty of characters they could have built up (Belmnot, Velonara, Heccular, Benthor Iceshard, Gunther Arcanus, etc). Instead, we’re getting stuck with a book character with no real ties to the faction set up as the faction lead. It’s honestly quite frustrating.
(It’s also frustrating that this all effectively ruins SL for the Forsaken. Here we are with the 2nd Death themed expansion, and yet again the death themed faction is sidelined. forsaken haven’t had a good story in years, and it sucks that this opportunity is likely going to be blown rebuilding the faction around Calia.)
EDIT: A final bit that I should have added earlier. She’s replacing a character that’s been leading the faction for 15 years. It’s very jarring to go from Sylvanas to a character that has only existed in a very minor role for a single expansion, especially when Sylvanas’s role in this expansion was poorly received.