A bair or a DownB used to be guaranteed while the opponent popped into the air out of the stun animation. The opponent, whether at 0% or 200% would pop up the perfect height(very floatily, too) for a flipspike or SH dair. DI doesn't allow one to escape. You used to be able to do this by reactions; once you see the stun ending, you can quickly input your move. as you know exactly where your opponent will end up.
I just did some testing in Bbrawl on ROB; for some reason, you can now DI the release animation of the Dsmash. This used to be impossible (or so minimal it didn't make a difference). The opponent can now easily DI his way out of what used to be a guaranteed followup. This drastically reduced ZSS' KO potential, especially at higher damages, and at lower damages, turns what used to be a guaranteed off-stage spike to an unreliable on-stage spike (AKA useless).
Also, how much can you lower the KBG without rendering Fox useless? Right now, it gets more difficult to get followups at high percents. If the DI issue was fixed, it should still be a guaranteed followup, but it would be harder. Might as well keep the KBG as low as possible.
You can see some examples of the guaranteed bair here:
http://www.youtube.com/watch?v=_FLq2Kl4jH0&feature=channel_page
Go to 3:40, and there are many examples. You can ignore the Dsmash -> UpB combo; it's techable by competent opponents. But notice how he ends each combo; the opponent pops into the air, and he jumps up with them for a guaranteed bair. In Bbrawl, these would not have been guaranteed if the opponent was to DI + SDI away from ZSS, especially at higher percentages.
While I was at it, I tested ZSS' 0-death infinite on ROB. Stun>Footstool>Dair or DownB>repeat - it's still in. It might be necessary to modify footstool properties to fix this though, which I'm presuming is difficult.