My Blue has been trained to wear leashes and harnesses. I recommend, though, that you only use these as another way of restriction while holding them while transporting them; dont walk with them on the leash. I say this because they are dangerous for rabbits and can be life threatening. Sarah mentioned that one user had to have their rabbits leg amputated because it got caught in the harness and no one could get its leg out in time before it freaked out and broke its leg. On a leash and harness while not being held, a binky can cause the rabbit to sprain or even break its spine, leaving it paralyzed or at the very least in a lot of pain. If they dash while on the leash and the leash reaches its maximum length during the dash, the rabbit can permanently hurt its shoulders, spine, neck, legs, or feet. It can also cause death if the rabbit jerks around on the leash. Walking your rabbit while its on a harness and leash has far too many cons over pros. If you decide to do this, be aware each time you are playing with fire and might not come home with your bunny because of one minor uncontrollable error.
But, if you want to harness (no leash) them in a controlled pen area, sure, go for it. A harness, if the rabbit is comfortable in it, can be ok in a controlled pen area as a back up in case the rabbit gets out. Obviously, if someone sees a loose rabbit outside in a harness, theyre going to know its a pet and might feel more obligated to call in for help rescuing it. Also, as I mentioned before, its a good tool (if the rabbit is comfortable) to use while holding your rabbit. Its easier to hold your rabbit by holding them, but also having one hand on the harness. If they try to pull away while in your arms, holding the harness acts as an easier (and safer) thing to hold onto to keep your rabbit secure in your arms. I used to use this tactic when get Blue used to being outside with me, and now hes super comfortable in new places while being in my arms and doesnt need the harness any more. But, those are the only two situations where a harness is safe enough to use. Outside of these two situations, there is a high risk of pain or even death for your rabbit.