Horus had literally half of the forces of the Imperium working for him AND all the daemonic powers of chaos, and lost. He was fighting a war that wasn't just on equal footing, it actually favored him, and he lost. Abaddon was always fighting a very different war. The imperium won the civil war, inherited all the power and resources of an entire galactic empire, one with a clear hierarchy of command. Terra issues orders and they just happen. Slowly, ponderously, but also inevitably, and without meaningful resistance. But the forces of chaos? First they lose the war, and with it their homeworlds, forgeworlds, ability to recruit or build power, they suffer further losses in the scouring, and then the forces left over turn on each other in the legion wars within the eye. Abaddon's black legion only rises to prominance in the wake of that, and Abaddon has to be constantly contending with rivals from other legions without anything like the brainwashed hierarchy of imperial authority forcing them to follow him, only his own personal power and charisma.
The problem with the old fluff, is that the GW writers kept trying to frame Abaddon as darth vader, the Black Legion as the stormtroopers, but they're not that, and they never fit that role in the 40k setting. Not at all. Chaos in the 40k era isn't the galactic empire, the Imperium is. Chaos were the rag-tag rebels, getting by on ingenuity and individual competence rather than mindless servitude and weight of numbers. When they wrote abaddon as tossing countless lives into meat grinders to achieve minor victories or executing subcommanders who failed to achieve impossible assignments, he just came off as stupid, because if you think for three seconds about the setting, he doesn't have those resources to burn. Abaddon simply cannot be Darth Vader. At least, he couldn't be pre-8th edition, where the imperium is broken in two and chaos has much more free reign and actually holds territory outside the eye.
but whatever, I'm getting way off topic here. The point is, Abaddon is another character I very much want to like, but it's hard due to how poorly he has been handled.