I have a small Yuzu that survived the winter outside in the ground in the Pacific Northwest (Olympia, WA, zone 8a). It was a colder winter than normal, and I only covered it with a paper bag and put a gallon container of water under there during the coldest night when the temperature was forecasted to drop down to 12 F in the very early morning. The lowest I actually measured was 19 F in that spot, and that was about 3 and a half hours before it was supposed to drop to the low point, so it's very well possible it never actually reached anywhere near 12 degrees in that spot.
After that it got completely buried in snow for a week.
It survived, lost half its leaves, some of the top branches died back, and it suffered severe bark damage on the biggest main branch at the bottom. But in late July it started putting on a flush of growth. Amazingly the leaves from last year have turned a healthy green color again, the leaves recovered.
It's on the south-facing side of the house, in a sunny spot.
I also planted two Yuzu seedlings as an experiment. One, in a shadier spot, did not survive the winter. Another, planted in a garden in a downtown area, surrounded by a courtyard and in a sunny spot, managed to survive unprotected. It was killed back and all the branches died, only the little trunk was left. All the leaves fell off, except for one tiny little green leaflet caught between the two main upper branches, but even that eventually dropped off in June. The seedling was only 5 inches tall, got killed back to 2 inches. It now looks like it has recovered to about the size it was last year. It was unprotected, except for being buried in snow. I tried to give it a little water every few days during the dry season.
I have posted pictures of this in another thread.