Chickens and rabbits share many of the same disease. Folks with chicken houses (large ops) at one time tried using rabbits to keep the chicks up and moving. (each time a chick gets up it will feed) Disease was the main problem and put a stop to that and (last I heard) went back to people walking the house once an hour.
Chickens like to peck (just a fact) and eye/ear injury to the rabbit has been a problem in the past.
Rabbits are sometimes territorial ... Just as a chicken can hurt a rabbit, the same can be said for the rabbit doing injury to a chicken.
Then we get to the feed ... They have a very different diet needs with the rabbit coming out on the short end of the nutrients stick.
Just some things to keep in mind and as always this is IMO.