The worse thing is handling the variance and discipline . I know a few counters that makes the same mistakes from time to time. I have messed up a few times even if it's not in any serious degree. Examples would be playing when table starts to get crowded, disregard heat , dealer is clearly giving crappy cut , etc....GL... it is not hard to count but hard to play long term and earn a decent amount. Of course it REALLY DEPENDS on the person !