I did some preliminary reading that there is something inherent about chocolate that causes it to harden ice creams more. I have yet to look into this beyond offhand mentions I've seen about it in some books and online.
My one chocolate trial did harden up a bit more than expected at 24hrs although nothing that couldn't be resolved with a bit of time at room temp.
Sadly being away for work so much and going on vacation soon I haven't had much time to experiment with the Ninja