I recently purchased the EOS R with the lens mount adapter.
Has any used a third party lens with the adapter, and, if so, were there any issues?
I am thinking of purchasing a Tokina 24-70 2.8 or Tamron 28-75 2.8 lens to use with my EOS R.
I have 2 Canon EOS R6 and have used both with the adapter and 3rd party lenses: specifically:
Sigma 150-600 Contemporary and the Sigma 60-600 Sport lenses. Both bodies and lenses performed flawlessly.
I have also used the adaptor with the following Canon EF mount lenses:
17-40L f/4, 24-105L f/4 MkI, 70-200L IS USM f/2.8 and f/4 (both MkII), EF 100-400L MkII, EF 70-300 (all 3 versions)
So I would suggest the risk of an issue is low, but in the end you need to try a specific lens out to be absolutely sure.