I'm in zone 5. The weather here is historically violent and unpredictable, and right now, it is even more so with wild swings in temperature. That said, I've had no trouble keeping my trees healthy and growing well. Winterization is a concern. Everyone here works out some kind of solution. I've used a homebuilt version of a polytunnel, but at this point, I have a 6-car garage that doubles as a workshop and winter storage for my trees. There are both heated and unheated sections of the garage, so I can, with care, have bonsai that are somewhat out of the zone 5 range.
The main problem I've had with ume is hole shot fungus, which I control with liquid copper. I've had no other health issues with ume.
While the pot is small, in relation to the rest of the tree, it is not as small as the picture leads you to believe, nor is the top of the tree as large as it may appear. The purpose of the photograph was to show the flower and the whole tree, and the best way to do that is to use a wide angle lens.
I've discovered that if two pictures are taken of slightly different rotations of the tree, and if they're displayed side by side, you can see more of what the tree and its flowers are doing. The pictures tell a more complete story. These shots were taken the 9th of January and the ume is in fuller bloom than I expected.
The flowers are gone, the ume has been repotted, and now new leaves are pushing through to the sun. This phase of the ume life cycle is not often photographed, but it is one of my favorites. I saw a picture of an ume in this phase for the first time in Peter Adams' book, The Art of Flowering Bonsai. To his memory.