I have spent a great deal of time soloing the new Warbands and have noticed that we are unable to dismount the warband leaders in the new Wildermore instance. I get an IMMUNE display when a crit is supposed to dismount while in riddermark stance.
My experience has come from being able to solo every Warband including Shaguk and Conog in the new instance. These two mobs take an extended period of time to solo, ~20 minutes during which I am able to spam our two dismount skills over and over again while in Riddermark and yet they NEVER have been dismounted nor have I dismounted any other mounted warband leader in the new instance. I can however always dismount the adds.
I do not know which classes all have a dismount skill but I have verified the Hunter class is able to dismount warband leaders so it is not an issue of the Warband leader being immune to dismounts but rather immune to captain dismounts.
I have not checked as of yet if we can still dismount any of the old warband leaders but it is fairly evident that captains have received a nerf in this regard. It may be due to the fact that 1 of our dismount skills can be an ae and they felt it was too powerful if we could dismount bosses as well. This is just a guess of course but it does appear that this is an undocumented nerf to this class that I would like others to verify.