Yes, the Canon EOS R50 works with the SanDisk Extreme PRO SD UHS-II V90 128GB — with conditions (see below).
Canon EOS R50's SD slot is UHS-I only. Canon's own R50 manual lists only "UHS-I cards supported" for this slot, no UHS-II mention. SanDisk Extreme PRO SD UHS-II V90 128GB is rated V90, a UHS-II-only guarantee that does not survive that fallback -- no UHS-I-mode or U-class figure is sourced for this card, so real sustained write speed here is unconfirmed. Verify before relying on it for 4K30.
Conditions for compatibility
Falls back to UHS-I mode on Canon EOS R50's SD slot -- SanDisk Extreme PRO SD UHS-II V90 128GB's V90 rating requires UHS-II and does not apply here. Verify real sustained write speed in UHS-I mode before relying on it for 4K30.
“SD/SDHC/SDXC memory cards UHS-I cards supported.”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)