<--- This guy's CDX works fine, I burn using Verbatim 80 min 24x discs and TDK 80 min 32x discs. I burn at my recorders max, 16x. Works like a champ. I've burned ISO + MP3, plus I've made Bin/Cue backups of my poor old originals, and straight CD to CDR backups. All methods work fine. I might suggest you are missing some ingredient in the burning process that is making your CDs not bootable... Dunno.
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.