I've never had a problem focusing a (Nikon) WA. One must only pull the split prism to the point you want focused and turn the camera a bit, if necessary, to get some angle across the split. Focus, compose and done. This works for me down to f/3.5, the slowest WA I have (28mm non-Ai). I'm 58 so we'll see what my eyes have to say about this going forward. Additionally an RF, unless I use an external viewfinder, does not show me the 'distortion' a WA provides, only its FOV, an SLR cannot avoid showing both. I am often after just that and it helps composition to be able to see it.
Having the muscle memory to pull that split prism around, now that I own an M3 one would think it would be easy to remember that I have to pull that split image around too. But I'm having a hell of a time remembering that when things are moving fast (My kingdom for a ground glass!). This might help explain the popularity of 35mm lenses for RFs; the DOF makes it easier to go all HCB on it.