One tiny thing to add: Misa mentioned this in passing, but Monks do not have a "downside", in the form of the Syker's big-brain/small-body dichotomy. I believe the reason for this is [i]scale[/i]. A Monk might be able to live to 300 on a sip of water a day, punch through walls and leap tall buildings with a single bound... But a determined Syker can erase a [i]city[/i] by manipulating memories, and can exert their will over large swaths of the population, all the while peering into the future to make sure they're on the right track, and taking out rival seers as quickly as they are born (and that's just the stuff Hanged explicitly mentioned). 

A powerful Syker would be a [i]very[/i] dangerous villain, but they also get the greatest drawback of the three Sepcarim. Considering what they're capable of, whether it's really a drawback or not is debatable, but the intent is there.