UC writes are very inefficient/slow. They are synchronous and not batched into cache lines. Oldschool GPUs used a UC-adjacent mode called WC (write combine) which was almost identical to UC just with adjacent-writes batched into full cache line writes. I think newer GPUs have dedicated DMA engines for copying to GPU memory that isn't mapped into the host address space but I'm not really familiar with these drivers.
For anything that isn't MMIO, you would prefer using ordinary WB caching and non-temporal stores to avoid populating the cache instead of UC mode.
For anything that isn't MMIO, you would prefer using ordinary WB caching and non-temporal stores to avoid populating the cache instead of UC mode.