Returning player here (out of the loop for 1+ years).

Ever since Kaiju released his video post-class reboot about Succession Sorceress, I haven't heard anything else that as changed since then. Is Succ Sorc so bad that nobody actually plays or cares about it anymore, or has it changed ever since the class reboot? Or is she actually pretty good, just that nobody plays her for one reason or another? What is the state of her now?

There are people playing succ sorc in both PvE and PvP. In PvE succ sorc is better than awakening. In PvP it is different but I think both are viable. For more precise info you should check the sorc discord server which is linked in the official BDO discord. 

Hmm I see, I was watching some tier list videos, like this one, which painted succ sorc in a pretty bad light: https://www.youtube.com/watch?v=zmCNP5-ORyU&t=318s

But then there's a video on Arena of Solaire, where they say succ sorc is definitely better than awakening, but she's not played almost at all in NA and only in EU? https://www.youtube.com/watch?v=K5C_susxRzM&t=8393s

Is succ sorc different in EU than NA, and do people just not have a good consensus on her because she's so unpopular?

its not really that its bad, sorc is giga scary and has amazing outplay potential, its more that drakania, zerker, archer and other fotm classes exist and devs are rolling their thumbs instead of balancing.

Succ sorc is definitely viable if you like the playstyle, but compared to the meta classes, she's definitely in a weird spot. She's no tamer or awk ranger that will iframe grab and blow up and then iframe away.

Her pvp damage is still a bit undertuned and most of her protected abilities are forward guards which is great for PVE but risky in PVP as she has a low number of super armor. 

Stamina issue is still a thing specially if you're facing iFrame classes and since she has neither S or Q block nor anything similar to awakenings nightmare, if you're out of stam then you're dead because her small distanced dashes become even smaller and only 1 dash has protection.

These are some of the reasons people don't play the class much anymore because there are better options available.

These are the big changes of the class after Kaiju quit

1. Prime Abyssal Flame charge is now super armor, damage is increased but she cannot spam it like before.

2. Prime version of Darkness Released was added. It's now a flexible skill because it's forward version(W+F) is a knockdown and back version(S+F) is a forward guard.

3. Prime Rushing Crow is now fully super armor instead of first half being SA, second half being CC.

4. Some new cancels have been added like Ultimate Shadow Eruption cancel after Prime: Dark Flame and Prime: Claws of Darkness. 

5. New Magnus Skill which replaces the input of Signs of Agony. It's an awesome skill.

6. Cancels added for the 3rd Rabam skills (Shadow Hellfire, Shadow Wave)

In addition to what Redeem said, Sorc Succ is also very accuracy-starved due to any modifiers missing on its 2 best dps skills (violation and black wave) & its accuracy/evasion (de)buffs being tied to wonky skills that are not great for Pvp. If you want to be pvping in open-world or rbf, it's a very costly class to become effective (gear-wise) since accuracy is the most expensive spec in the game rn.

The class still has several dead (or should be dead) skills:

-Sinister Energy {ded}
-Flow of Darkness {should be ded}
-Absolute Darkness {should be ded}
-Beak Kick {should be deadest}
-Bloody Contract {ded}
-Sinister Omen {deadest}
-Storming Crow {ded}
-Shadow Kick {they d teased us with TP2Back Prime:Shadow Kick only to take it away}
-Shard Explosion {unfortunately ded}
-Prime: Agent of Abyss {ded...gives 200mp?? and 0.4 iframes}

Otherwise, in NW and AoS, it's performing as well as it can, nothing close to the effect DOD>Grim>Crows Awa can perform. Outliers can perform very well on this class [in nw and aos] due to the fact that it's very rare to meet in the wild and they do not know its animations and protections.

But overall, if these are the last changes PA will do to it and consequently buff Sorc Awa in the future (which they will), it's a more or less gimp spec. You could play Awa and get the same kit (minus black wave and long calamary) in exchange for a whole additional Awa kit. You could also play orange sorc (Succ Maegu) for even more effect. Or it's -somewhat- male counterpart, Sage, and just makes ppl break their keyboards in pvp with his troll movement and exquisite (up to) 51% accuracy/evasion buff/debuff along solid skill modifiers that make sense.

What I've realized trying to advocate for this class is:

A. The community has PTSD from Scythe and projects it on Succession. It doesn't help that there are outliers in KR/NA/EU that overperform with this class making it look very strong.

B. Scythe is J's and PA's sweetheart and they will never ever ever ever make Succession anything decent that could pull away more than 5% of the players from awakening. This was proven by the fact that they took Kaiju's and several of this forum&reddit's ideas for actually finishing the class, and those same ideas were implemented as mechanics for Maegu and Woosa. Massive aess slap in the face.

I know I'm late to the game on this thread but just in case anyone else stumbles in here like me;

I love playing succ sorc. I tried awakening but it just didn't hold the playstyle I liked about sorc starting out. She is incredibly fun to play, and a very active class to play if you are looking for something super engaging. That being said though, succ sorc is hard to play (especially PvP) and definitely needs to be geared up to take on fights in PvE and PvP. If you can master her though then succ sorc is a formidable opponent. In PvP you definitely have to be calculated about everything you do while still maintaining speed  in your actions because you will get knocked down, and you will die if you don't. She is typically pegged as a low mobility class but I think it is because awakening is lower mobility, and not many people play succ. I won't tell you she is high mobility but I will tell you that you can really clear out some mobs with her.

She is definitely viable for all parts of the game. Just get geared, learn your moves/combos, and don't be afraid to be patient in your fights. She has a lot of i-frames (invincible ~1sec), super armor (take damage but impervious to crowd control), and a couple front shielded moves that compensate for her weakness, and allow you to jump in for knock downs followed by a bit better than average damage output before jumping away from retaliation.