In UEFA champions league, away goal rule still applies AET.

In CONCACAF champions league, away goal rule does not apply after extra time.

In my opinion, CONCACAF does a better job applying the away goal rule.

because:

each team plays 90 minutes as an away team
each team plays 30 minutes basically on a neutral ground.

During extra time, it sucks if

AC Milan and Real Madrid both tie each game 1-1 during regular time. Then the away team in the second game, AC Milan, scores during the 95'. Now Read Madrid has to score twice to win the game since AC Milan has the extra away goal. Why should AC Milan benefit playing 120 minutes as an away team but Read Madrid only played 90 minutes as an away team. Unfair to Read Madrid.

If you treat extra time as a neutral ground, Real Madrid needs to score once to draw and even the aggregate and force penalty kicks.

This is how CONCACAF runs things, and it is much better in my opinion.

And personally, I wouldn't mind if PKs were eliminated and players on the pitch were reduced during subsequent extra times with golden goal rule reintroduced.