Yes, the Canon EOS R8 is compatible with the Samsung PRO Plus 256GB SD.
Canon EOS R8 has a full-size SD UHS-II slot; this card fits directly. Samsung PRO Plus 256GB SD is V30 class. Meets the floor for: 4K30 (30 MB/s floor). Does not meet the floor for: 4K60 (60 MB/s floor). (Peak write 90 MB/s is not evidence of sustained performance; the V30 class rating above decides this.)
Limitation
Cannot record 4K60 -- requires Video Speed Class V60 or higher. Works for all lower-bitrate modes (4K30).
“29.97 fps 25.00 fps 23.98 fps IPB (Standard) UHS Speed Class 3 or higher IPB (Light) SD Speed Class 10 or higher”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)