Some food for thought.
The idea of a home and away series appears to be a design that is supposed to bring parity to an int'l competition. I always thought that there were two major flaws with the design in a club competition:
Away goals rule: In my view, this rule is garbage. I understand that it's a tiebreaker to circumvent having to go to extra time and possibly penalty kicks, but its intention is counter intuitive. In my view it does not reward a team that plays open attacking football on its home ground. It simply rewards a team that is better able to keep clean sheets at home. If you're tied on aggregate, and lose on the away goals rule, that means you must have scored more goals on your home ground- why not be rewarded for that ?
Extra time & penalty kicks This is an age old argument/problem and I will admit that there is no perfect way to solve it. I do think that in a home-away series, extra time & penalty kicks is advantageous to the team that plays at home in the second leg of the series. It gives that team more minutes played on their home ground, and the home-crowd advantage for the penalty shootout....
All in all, I think the home & away series is advantageous to the team that plays at home in the second leg. They have a tactical & psychological advantage as the second leg is contingent on what happened in the first leg, (I think its easier to execute tactics on your home ground)...... they are less likely to concede goals at home because psychologically the concept of away goals seems to be more of a consideration in the second leg than in the first, and with extra time & pks on your home ground, the home team is the driver's seat if tied on aggreagate after 210 minutes.
Of course no empirical studies have been done to support this argument, it's just an opinion, and I admit I don't have the answers...just wanted to put it out there and see what other people think.