Hasted CDs typically mean that the overall ability ratios stay similar, because you just get to press everything faster thanks to shorter CDs and more resources. CDR could end up pushing out abilities because you simply don't have the time to press then anymore, because more important abilities come off CD faster. In the extreme case you end up with something like your FoF example. It would also very easily become very breakpoint-y, which Blizzard doesn't like.
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Yeah, that's the issue. The stat in itself wouldn't be too bad(outside of breakpoints), it's the things they'd likely need to do to balance it that'd be confusing.