Some serious problems with the mercurians... First Every time a mercurian or an infernal is summoned into the world, blight hits... not sure if that's a bug or not, but it's really crippling.
Second, and I think this one has been mentioned before, but Basium is the only Mercurian leader that successfully spawns. All other die instantly when the mercurian gate is built. I know this because, since I wanted to play as the mercs, I used worldbuilder to remove the gate and add enough cash to rebuild it next turn. This resulted in a total of 5 blights including the normal one at 30 turns, the infernals, and two failed merc leaders before I got Basium.
Third, as far as I know, mercurian leaders are supposed to perma-war with infernals, which was not the case in this situation.
Lastly, whenever a good civ had a unit die, an angel spawned as per normal, but a python exception would pop up saying something about change unit experience variables didn't match up. (I'm at work so I can't give you the exact wording)
If I remember correctly, the angel's experience is supposed to be based off of the unit that died isn't it? If so that's what's broken there since all of my angels start with no xp.