I have a 4'x8' utility trailer with the 12" wheels. I routinely tow it at 70 mph and have for years. I check the bearings and grease them every year.
Before this one I had a same size trailer with 8" wheels. Towed it the same way for many years until a guy ran into the back of me and crunched it. Replaced the bearings after several years when one of them got a little worn, but not failed.
Those little wheels scream at highway speeds but the hubs have never gotten close to being hot. I always feel them when I stop for any reason.