I've seen and experienced Atlas coupler failure due to zinc rot but I haven't experienced problems coupling to Kadees...yet. I do know that the older Atlas couplers had several issues -- stiffness of the centering spring, having to slam them together that prevented coupling, etc. So far, they've coupled to both the 800 and 740 series Kadees just fine, subject to having to hit them harder.
I also experienced a problem of the long-shank couplers flexing with my 89-foot piggyback flats. This happened under heavier loads. I replaced them with Kadee 746 long-shank couplers which are stiffer than the Atlas plastic shanks.
I've been looking at replacing Atlas couplers with Kadees on other rolling stock and on conversions