embryodead
Caliph
Mongols certainly have not been nerfed. I don't know what's causing it, but it's most likely related to stability, because civs still lose cities, but seem to be collapsing less often from this. There was a stability fix in last patch (Courthouses were not giving the bonus), for instance. Also something could have gone wrong with civ bonuses when they were moved to Python, but if that happened, the consequences would have been much more noticeable.