I would personally prefer a group fitness class outside. Not as an instructor but as a client. I’m not group fit certified, so I don’t really know much about designing classes.
Just being a prude here, the only thing that I might worry about would be that I can’t be sure that there would be video record of what happened if someone injured his/herself. I’ve got a degree in Justice Studies, so I tend to worry about legal liability a lot. I always want to make sure that I’m covered! It only takes one person to ruin a career!