Hot take: Abaddon is a better Warmaster of Chaos than Horus

First off, Abaddon is the only one who's actually unified chaos. Horus did not unify chaos. Chaos had already unified by itself because of the existential threat the Emperor of 30k posed. Horus was a puppet that Chaos rallied around, but he never really unified it. Abaddon on the other hand has actually forced Chaos to work together himself. He became warmaster at a time in which there was no existential threat to the chaos gods that caused them to unify together, and instead chaos went back to its usual disjointed itself. He's actually forcibly unified the disparate chaos legions together into a coherent force.

A lot of people will mention how Horus did more in a few years than Abaddon did in the 10000 years before the fall of Cadia, but that's because Horus had a lot more to work with. At the beginning of the Heresy, he had 9 fully unified legions that hadn't degraded to the level they're at in 40k. He had command of 9 primarchs who also hadn't gone completely insane yet (minus Angron and Curze who were always insane) too. Abaddon on the other hand inherited a nearly destroyed 16th Legion, and instead of having 9 unified and disciplined legions, he had to work with 9 mostly shattered legions whose members had gone completely insane. He can't even make the Black Legion into a fully unified/organized Legion like the legions of old, as the traitor marines have degraded far too much at this point and the most he can unify the Black Legion is by making it a loose confederation of warbands. Even though he has the approval of all the daemon primarchs, he can't command any of them like Horus was able to. The most he can do is use Angron as a tactical nuke, but he can't actually command any of them to do anything like Horus could.

Additionally, Horus had access to half of the Mechanicum (and thus half of the Imperium's industrial base), half of the Imperium's mortal military, and occupied a massive chunk of the galaxy, whereas Abaddon has to make do with what he can scavenge and only directly controls territory in the Eye of Terror. And finally, while Horus became Chaos' champion was chaos was beginning to become ascendant, Abaddon became champion when Chaos was at its weakest. After the Emperor killed Horus, Chaos was severely weakened. The Gods had poured massive amounts of their power into Horus, so his death severely weakened them. The traitor legions were stuck in the Eye of Terror unable to leave, and had started killing each other. Abaddon ended the Legion Wars, created a new Legion from the ashes of his destroyed old one, and was the first Chaos Space Marine to lead forces outside of the Eye of Terror. He's arguably the main reason the traitor legions continued to be an actual threat instead of a minor nuisance after the Heresy. The fact that he nearly caused the Imperium to collapse (which would have just happened outright if Guilliman wasn't resurrected) while having so little to work with is incredibly impressive.

Abaddon might have achieved less in 10000 years than Horus did in 7, but he had significantly less to work with and started at a far worse position. There's no existential threat for Chaos to unify itself against, so he has to actually put in the work to force chaos to work together unlike Horus.