It certainly feels like they
A quick look at the stats and united have actually had the third highest number of yellows for the last two seasons, imagine how many more they'd have if they didn't get such lenient treatment.
And you can bet most of their cards do come late in the game when they don't really matter and the refs are just trying to make the card count look more even and respectable, I can't remember the last time I saw a united player pick up a yellow card for a first offence early in the game.