I've said it before, this game REQUIRES thought and prediction to be good at. You can't just hit someone, then go through a routine combo you've practiced so much its now instinct to perform. Combos are still possible you just can't hope to do them the same every single time. If you can't predict what you're opponent will do to avoid being hit, then simply put your not going to hit them often. If you can't predict how your opponent is going to attack you, then your going to get hit extremely often.
Nothing is ever guaranteed in this game, and thats why I like it. You have to work hard and think a lot to get any good strings of damage on an opponent. When you do get a 0-50% string of damage, you can be happy that you've outsmarted your opponent well.
You want to see a truly broken defensive engine? Play SC3. That community just died because of it. It was apparently broken enough that when they designed SC4, they added in OHKOes against people who go high in defense.
My Creations in Modnation Racers! Check zhem!
http://community.modnation.com/en-us/players/IvoYaridovich/creations
