Depending on temperature, umidity and other condintions, the CDX's laser might get less sensible with time (considering most CDX'es were build 7-8 years ago...). CDRs are usually harder to read than pressed CDs, mainly if you record them at high speeds. I burned Silpheed (my first SCD burn), in a good media, at 12X. It played fine back when I burned it, but it gave me lotsa read errors past stage 6. Now it hardly gets into the game itself.
I got my CDX used, found it buried in some store, box-less, cable-less. I have no idea the kind of stress this baby suffered during his life.