If you were prepared to cancel on block (so you delayed the cancel purposely by 5 frames) and they get hit, you will still cancel the skrc 5 frames later (or more, if you didn't get the perfect delay). This will prevent some combos from happening because the hit advantage is not always enough to guarantee a follow up if you waste 5 or more frames (for examples 4skrc, f2; f2skrc, f2; f2skrc f3; f24skrc into anything).
If you were prepared to hit the opponent but they block, the situation is even worse because the cancel won't come out at all:
I usually input SKRC, ffblock right at the end of the string, but for f3SKRC and 4SKRC I input everything fast and then hold the Shadow Kick a little before cancelling it