I'm an avid Zarathur player and I'd love to weigh in.
To give some context, I've won multiple store championships with him, took him to worlds, he's probably my second-highest played warlord (Behind Nazdreg, I like to test Ork stuff a lot) but competitively, he's my go-to guy., but chaos is not the weakest faction, Ork is. Chaos is just close behind, and it's not just because Orks are their ally.
The problem with Chaos seems to be because they get held back by his ability, however, they are not shy about giving this damage to their allied factions (and for cheap)
Look at SnakeBite Thug. If Snakebite was a Daemon people would probably cry foul. Paying 2 for a 4-ATK unit (that can probably survive to the second round of combat) is crazy good value. Even before the well-costed, high-value 'Ard Boyz Zara was competitive, yet vulnerable; but now her has his own personal bodyguards with high health and 3-attack and 2 command. Great card for Zara, game-changing even. With his Daemon reducers he can afford to put out the 'Ard boyz and some other units, as well. Add in Ammo Depot, Caius Wroth, Ork Kannon, etc etc. you have the conceptual beginning's of a deck that seems incredibly strong on paper.
Ork's have some monster ally cards but suffer from average sig-units, below average events and over-costed supports. So why with such a strong ability is Zara still not hitting top-tier?
My opinion: The culprit is low-value units and the inherent difficulty in playing Chaos.
Zara's deck, sig sqad, tricks and strategies are quite simply not as easy as everyone else's. Chaos is full of trickery and janky-ness. sacrificing units and using Slaanesh's Temptation without knowing their hand and casting Warp storm on your own units, and a bunch of other non-traditional moves make for a hundred little choices in each Chaos game; there are no "optimal plays', only 'good guesses' based off of tons of experience.
Mix that with some sub-par 3-cost units, some less-than-average 2 shields and you have the makings of a very ork-like deficiency, specifically: Chaos needs some better cards. Because when you pay the same 3 resources for a Khorne Berzerker that a SM pays for BAV, you cannot afford to lose any more value when you've already lost value playing sub-optimal units.
In a game where Gift of Isha, Drop Pod Assault and Preemptive Barrage can win games in a single combat round, sub-part units will not win a game for you.
Eldar, Dark Eldar, Space Marines and recently Astra Militarum can do something that Chaos, Orks (And in my opinion, Tau) cannot do: They can make mistakes and still recover from those mistakes with high-value events and units. Chaos, Orks and Tau cannot do that.