Extender EF 1.4x III compatible with EF mount?

[gears4steam]

I'm looking to see if there is any benefit to mountiig an "Extender EF 1.4x III" to a "canon eos r lens mount" for my Sigma 14-24mm for Canon EF, with a Canon EOS R body. 

Any takers?

[rs-eos]

None.  It's not compatible with wide angle lenses.   Even if it worked, it would only give you a 20-34mm equivalent lens.

Refer to Instruction Manual for a table on usable lenses.

[Tronhard · Accepted solution]

Leaving the focal length aside, if you are using an EF extender, then that adds yet another layer of connections between the lens and the camera body.  That is problematic - the fewer connections the better.  Furthermore, extenders have compatibility complications even with Canon lenses to give full AF and exposure settings. Lord knows how it would work (if at all) with a 3rd party lens.  As Ricky wisely put it the extender would give you very little advantage on that focal range in any case, if in deed it would physically fit.
If you want a focal range of something like what the 1.4 extended would give you, it would be cheaper, simpler and better quality to get an R-series lens that is meant for the MILC R-series cameras like the RF 15-30, 24-50 or the 24-105 variants.