If two equally talented men got into a fight, one trained in Aikido and the other trained in any relevant martial art used in MMA, it's almost certain that the Aikido fighter would get crushed, save some miracle.
The Aikidao operates on surprise. On the first attack, it certainly will be affective. In a street situation, it is very affective because most fights will end if the initial attack ends so abruptly.
Anyone with any degree of levels of Martial Arts will not find them facing each other since that's completely against the Crede. Usually. Case in point is Steven Seagull who is a bully.