Akuma's ultra might do around 70% to Seth, the character with the lowest health in that game lol, but it doesn't do that much to everyone. Also Ultras can only be done after you take damage or absorb focus damage as you may know so you obviously can't do an ultra anytime near the beginning of a match, whereas Cyrax can do 80%+ with only 1 bar which you get at the beginning of the fight. There is an obvious big difference between getting out 70% to some of the cast only after you get beat up enough to get your revenge bar up and getting 80%+ with the very first combo of a match.
Another thing is you probably aren't going to get use your ultra twice is one round, it is possible but in Akuma's case don't count on it as he has relatively low health and if you do get it twice one of the two times the ultra probably won't be at its fullest but in Cyrax's case every time he hits you, given that he has at least one bar, you die. Again, obvious big difference between getting to do this big damage move once and maybe twice in a round and getting a high damage combo out every time you touch your opponent.
