Having used both phones, I personally feel that the Mi 11 Ultra is the better and cheaper phone. I paid $1045.00 for my phone (12/256), whereas the S21 Ultra cost $1249.99. I would agree that the zoom is better, but photos are usually better in daytime shots and better in night/low light scenarios. The video stabilization I feel is also better. Also the bootloader can be unlocked on Xiaomi phones here in the US, wheresa Samsung can not. All phones seem to have shortcomings in certain areas and these two phones are not exempt from that.I'm neither in blind love of my Mi 11 ultra. But your main point is: Samsung did a better job on their software updates than Xiaomi?
Let's hope Xiaomi will release better software soon or custom roms to fix it. And well, when i have a S21U in my own hands i will compare them.
Thanks for sharing your experience.