Canonically Basium despises Sucellus, whom the Illians also hate, so they are not such a bad match.
An Evil civ following the Council of Esus summoning Basium also makes sense, as Esus (despite being evil) does not care much for demons or the undead and actually led the Svartalfar to fight against Tebryn and Ceridwen in order to save the world from Armageddon.
The Overlords use both demons and the undead and are strongly influenced by the supremely evil Hastur, Lord of Nightmares, so it is canonically nearly as incompatible with the Mercurians as the Veil.