The Canon EF mount uses a lens flange distance of 44mm vs 42mm of the FD mount. This means that the lens could not focus at infinity unless the adapter has a corrective lens in its design. Most of these adapters are of very poor quality and will degrade the IQ of the lens.
Depending on what lenses that you have it may be worth changing the mount of the lens using one of the Ed Mika adapters.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
04/20/2023: New firmware version 1.4.1 is available for EOS R3
03/30/2023: New firmware updates are available.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM