CS =/= CT. The cancel window is really late, almost when Ragna's downward swing is about to hit. Really, try it out in Training, if you ever go there.
That is true for any non-airtight blockstring. It's not exclusive to 5D(1) DC.
Ok then, go smash your face against the controller to counter it. I'll just not dash cancel and get a free CH. I don't think you understand what I'm talking about, but then again, that might be my fault for not explaining properly. I'm gonna address that in my reply to Titanium, stay tuned.
You forgot to factor in first active frame hit, and blockstun. If it hits in the first active frame, it's 12+4 = 16 frame gap. According to frame data, 5D is a level 4 move, so blockstun is 18 frames. Factoring in IB, it's 18-5 = 13. So the opponent has 16-13 = 3 frames to do something in case you don't dash cancel. Frame data says a dash cancel is 22 frames of recovery* (*dunno what that means, but I'm guessing it means it lasts 22 frames), which is bound to be more because of the delay in dash cancel (I'd say around 13-14 frames after the first active frame hit). The thing is, that the dash cancel has no voice sound cue (only that dash sound, which is mostly masked by Ragna's 5D shout), and when done, even some of the "darkness" effects come out. So the opponent has to rely on only visual cues of Ragna running AFTER you dash canceled and do a reversal on reaction, because otherwise they'd be expecting the 2nd hit of the 5D, or something else, because there are other options after 5D(2). So factoring in average human reaction time (according to yahoo answers is 0.25s) which is ~15 frames, that'd mean they have 23-15 = 7 frames to do something, IF they were REALLY alert (not accounting when they actually see the visual cue). I'd imagine that if they were gonna do a DP anyway, they'd be too far away since the blockstring itself prior to the first hit of 5D would have pushed them back a bit, since no Ragna should intentionally start a blockstring with a 5D (just a guess, gonna have to get home and test it with every non-Jayoku DP in the game to verify this). And you can always cancel with C ID yourself if you see them IB or feel they're gonna DP (make sure you have 50 heat before doing so), and not dash cancel.
That's the math of it, but theory =/= practical, otherwise all overheads ever done by every player would've been blocked, or DPed, everywhere, and all throws would've been teched, and elite JP matches would always be a draw with timeout. Try it out yourselves (which most of you didn't seem to do), and tell me what you think.
So what I'm saying is that it's something to DO to mix-up your opponent once in a while (hit/throw mix-up). I've never said that it should be relied on SOLELY, just that it's something that you can incorporate into your game, to keep your opponent on their toes because you have one more option at your disposal, and they should account for that.