Scriver also posted something like this but I thought I would add my two cents. So I decided to make a large party of 10 adventures with mounts (coinciding with the max unit size in dwarf mode). Thinking we were going to go on a big adventure. Not so, within the first two days of setting off, 7 of my party members had killed each other. They had seemingly all gotten into "violent disagreements", i did notice that my two morally questionable "antihero" members were the first to be killed off, implying its a personality incompatibility thing. They would even attack the mounts for no apparent reason, then others would join in. They would spit at one another over having killed each other and it just resulted in one loyalty cascade after another. In the end, I was stuck as single party member even though two others were left alive. They had both "concluded the agreement" and so I wasn't even able to tab over to them. I convinced one to follow me, but then he only acted as a normal follower and I couldn't play as him. So ya, party support seems kinda buggy at the moment. Maybe im pushing the limit of what Toady intended with this but idk.
Also, Mounts seem to have a mind of their own, when riding into battle I seem to have issues with killing one enemy and then having my horse run uncontrollably in the other direction out of fear, they also seem to wander around randomly at times which may be affected by the new Rider skill. Im considering adding the NOFEAR and CANOPENDOORS tags to certain animals.