One of the benefits of the 5E version is that it makes seven attacks: six with its longswords and one with its tail. So as the DM, I can quickly see, 'okay, +9 to hit, 13 damage'. I just repeat that six times. Boring, but easy to remember.
With the ToV version, it has two dagger attacks (9 piercing 7 fire), two mace attacks (11 bludgeoning 7 fire), and two longsword attacks (13 slashing 7 fire). So as a DM, I have three different sets of damage numbers that don't differ all that much. Why? I don't feel like the juice is worth the squeeze. It feels like an unnecessary complication of the statblock. It's also still boring, just now it's harder to remember.
The Bonus Actions are cool, and the extra reactions are nice (assuming you remember that this specific creature has five extra reactions to use, which maybe I'm dumb but I will almost assuredly forget).
The different weapons struck me as clunky, too. I'm already rolling 5-6 attacks, don't give me four different calcs!
I like the addition of a command action, but I'd like it even better if it was more
demonic. Demons don't follow orders. They don't do tactics like mortals do. The marilith should be pushing them to destroy, to damage, to sacrifice themselves in a ball of fury. Like, what if the movement had to be
toward an enemy, and instead of advantage on an attack, she granted them extra damage, or the ability to hurl creatures they damaged, or just an aura of fire (that could also damage the fiend she granted it to, why not?), or some sort of barbarian rage effect? It's currently just sort of a generic commander ability, but we could probably make it evoke the vibe of
a bloody DEMON GENERAL, yeah? You could play with the idea that the marilith is a pretty savvy operator by doing things like granting invisibility or flight or some temp hp or something that the front line brutes wouldn't think of, but which she knows will be more devastating.
The teleport change is pretty good. Gives it something to do with a bonus action and keeps that D&D demon vibe.
The fire damage seems...fine? Like, not exciting, but sure.
Not a fan of looking in two different places for immunities and resistances.
Reactive strikes me as actively worse than the 5e version, which lets the marilith do OA's as well as parries.
Y'know, mixed bag. I'm on board with them playing up the distinguishing characteristics of the monsters, but also, don't give me a generic commander ability, give me something that only the commander of a hoard of entropy-reveling forces of destruction and carnage would do!