Lad at work (Man Utd fan, surprisingly) is making a massive issue of this. I have some sympathy but as far as I'm concerned, the 2 advantages cancel each other out - Sunderland have 30 minutes extra to get a goal that counts double, but Man Utd have 30 minutes more in front of their home fans.
Besides, after 180 minutes if you haven't beaten the other team decisively, then obviously there isn't much between the teams and you can have few complaints about getting knocked out on away goals be it after 90 or 120 minutes. Tough shit.