The attack vectors I'm thinking of just come from the inherent complexity and centralization. I'm just considering the amount of damage that can be done with a compromised DA account for example vs a non directory environment.
It's complicated. Done right it can be more secure, not done right it's less secure.
I also only get brought in for problems for the last however many years, so I'm probaby a bit biased at this point haha.
I have had to tell companies they are going to have to rebuild thier AD from scratch because they didn't know what thier DSRM password was (usually after a ransomware attack). These are the sort of hassles I think about vs non AD.