U-3 is used specifically because we have so much historical data - it lets us compare how we're doing now to the great depression, for example, and up until a f

U-3 is used specifically because we have so much historical data - it lets us compare how we're doing now to the great depression, for example, and up until a few decades ago it was fine because all the various unemployment metrics were strongly correlated. Politicians eventually gamed the metric so hard that it's almost worthless now, so economists today mostly use U-6.