Damage depends on level of demonic mastery / bind demon as you enter a game, which you can lock in with +skills swap gear. Even a warlock with just 1 point into those skills can swap gear for +37 skills, and it caps out at 57 for a maxed out build.
The damage of all (melee) bound demons is about the same because the vast majority of their damage comes from that +damage and +% damage on the two skills. If you can have as much as +1600 base damage and +800% ed, it doesn't matter as much if a demon has 50 vs 100 base damage or a skill with +120% ed. But their attack rating does vary, and in PvP they have pretty lackluster AR and only conviction to remove anything from enemies. You can however enchant them, or have blessed aim on a merc instead of might.
an LLD warlock will have a very nerfed demon, because demons have their level brought down to player level and everything scales with that, on top of having less +skills. At most an lld warlock can have level 18 bind demon / 31 mastery