Well, technically other than the Tyranids he's problem the greatest threat because of what's behind him:Chaos. The Crusades may be lead by the CM but they are sponsored by the Gods. They create the warp storms, easy travel, safe haven, and daemon hordes that make these things possible. If Abbaddon wins, unlike Horus, the galaxy will burn and at no point in his insanity will he think he was mistaken. Horus, according to xenos sources, would have eventually destroyed humanity after seeing the error of his ways.I don't think Abbaddon will reach the same conclusion.
The Necrons represented an equally horrifying doom as wanting to shut off the warp from the materium to allow their gods to feast on mortal life forces. Fortunately for the Imperium, that's no longer an issue. Now it's just the Tyranids and Abbaddon.