I personally think its about 50/50 that a rabbit will be ok in a harness. Our Mini Rex is my ESA so he had to be trained form early on how to be in a harness. Hes fine wearing it, and is fine on a leash. We havent had him outside on a leash yet (he usually stays in his carry crate with me, or in my lap/next to me on the seat), but he is trained inside on a leash. While in the harness, if he gets nervous or scared, he was trained to run to me instead of just freaking out so i can pick him up. Our other two rabbits were not trained to wear a harness so neither of them like wearing one and both get nervous on a leash. Needless to say, they arnt put in a harness any more
Ive seen a rabbit that got scared while being on a leash. My friend was babysitting a Giant Flemish and they were told hes great outside on a leash, but the first day they took him out in his harness and leash, he freaked out and screamed. Everyone in the general area came out of their homes to make sure everything was ok. He was cuddled, had the harness removed, and spent the rest of the day [inside] easily spooked
As others have mentioned, you can get a pen to put outside so they can play safely. Youll want to watch them though so make sure they cant move the pen around or get out from underneath it. Well be doing this soon for our two buns who arnt harness and leash trained