Repetition and putting the combos into muscle memory is important, but you should also have it down to the point where you're comfortable with executing them in actual matches. When you're worrying about blocking and mixups it's much harder to stay calm and execute. I feel like repetition in training mode is important, but actual match experience is just as, if not more important.
As for practicing combos in training mode, I tend to break them down into parts like many people have already said in the thread. It's less frustrating than trying to nail it in one go.
