A very large part of why I left the last barn was one boarder. I’d been there for several years and had a good relationship with the other boarders and the BO. I could join group rides or not without fuss or offense. It was a low key, no fuss place with excellent care of my horses - even my needy one (I worked with the BO to figure put the easiest way for them to do his care).
The new boarder came in and was friendly, chatty, asked questions - more than I’d like, but I was friendly enough. Until I realized that all of it was designed to let her talk. She tell the same story five times (in ONE day), ask for advice and then question it or ignore it completely and start telling stories again, try and show videos or pictures, exaggerate everything (and then deny it all when it came back to bite her butt). It took a bit longer for it to be evident that she would lie freely, double down when called out, then deny ever saying it, and then promptly tell another blatant lie.
This boarder destroyed the peace in the barn and I wasn’t the only one trying to time my visits to avoid her. Unfortunately with two horses and a full time job I didn’t have a lot of flexibility. The barn was tiny, with no way of distancing. If she had no one to talk at (I was not an option by then) she would phone someone, put them on speaker and talk (no personal volume control in a tiny barn = no escaping it).
My job is stressful enough, I don’t need a stressful barn life on top. I didn’t complain to the BO, I moved.
Another boarder got me kicked out of a barn once. At last check that boarder had gotten at least ten boarders to leave either through harassment or persuading the BO to kick them out. That seems like a poor trade off to me.