I co-own Dance. There haven't been any problems per se, but as her breeder and I have virtually zero relationship, I'd rather Dance were mine outright. Her breeder is supposed to have breeding rights and I was supposed to show her and get her championship, but I gave up on showing her due to her temperament and how she was in the show ring when it was time for the judge's exam. To be honest, I'm not sure what made me decide to co-own... I don't think I put as much thought into it as I should have. But, live and learn I suppose. I've had a lot of "live and learn" moments regarding Dance. But anyway, I don't see anything wrong with co-ownership as long as you understand the contract and agree to it, but I also recommend only co-owning with somebody you're friends with or somebody you at least trust and know really well.
If I were to ever co-own again, which I'm not overly keen on, I'd want a co-own contract with no breeding rights. And no stipulations stating that I must put this title and that title on the dog. The only reason I'd even think about co-owning again would be so I could leave my dog intact for as long as I want to. Not sure of that type of co-own is common or not, but that's the only type I'd want to enter into in the future. But I'd strongly prefer just to own my dog and call it mine 100%.