This is a case of plot strength vs battle strength. They don't always tend to match up. Againwe don't know how the Final Aeon exactly kills the summoner. And once more they are not Yuna's Final Aeon. Yeah, they could kill her but so could the chocobo eater. Final Aeons don't have "rampage" trait when any old summoner is nearby. What said they were holding back more so in Jecht's case who said he won't? As we come to find out and from the history of Spira aeons can be defeated. The Final Aeon's strength comes from the bond of summoner and the person who became the Final Aeon. Its ability is to be able to pierce through Sin's armor. It could be that this ability is only for Sin. In the end there be no inconsistency because Yuna does not have a Final Aeon.