I bought an R6. I use it with the EF 500/4. Since I bought it the eos 5D mk 4 has stayed in the cupboard. I prefer the results from the R6. There are probably a variety of reason but there's, for example, no need for microadjustment for mirrorless lenses.
The adaptor is essentially just a spacer to enable the image to focus the image at the 44mm required by the EF system rather than the 20mm of the R system. Plus, of course, maintain the electrical connections between the lens and body. Go for it.
Eric
