Honestly, it is not a good idea for someone to fully predict a match based only on what happened in the first leg. A match in the first leg doesn't always guarantee that the team will win the second leg. Winning the first leg match can be lucky or likely based on some factors. In the second leg, everything can change due to various factors. Any team's performance can improve or decline at any time; all these happen because of one or two factors. In fact, this is the reason why many gamblers who bet on football matches lose because they always focus on what has happened to predict matches. But that is not supposed to be; we need to always consider present factors of teams before predicting their outcome.
We saw a lot of that happened last season between some tough clubs and like you said in the second leg there might be some factors that will deprive the team that won the first leg from winning. Factors like injury or red card in the match.