1.  I'm not sure if it applies to Slave Lords too, but if the leader (first unit) of an army is Men/Fallen, if you win all of the units killed count for bonus population.

2.  The new lairs with the new undead units count as Men/Fallen for population purposes.  However, the pre-existing Monster Skeletons and Banshee units don't (I'm sure about the Skeletons, Banshees probably are the same).  The new units are Undead race-type, the old units are just monsters with similar undead abilities but no undead race (ie. monster type rather than race type).

3.  The new lair has a Ghost/Banshee unit with no attack, that does nothing but cast Terror if you go next to her.

4.  I think the new undead lair units cast terror too much.  Sometimes you don't receive any damage, you just miss lots of turns because they cast Terror a lot.  I find the new undead lairs relatively easier because they cast Terror a bit too much.

5.  As already mentioned, the Undead cities don't have the proper undead garrison troops - for some reason they have been set to have Wraith garrison troops instead.