I think there are 2 key reasons why Juve has underperformed in Europe:
1) Mentality (tend not to take the game to them as much)
2) Lack of counter attacking threat.
In Europe you need that counter attacking threat. It allows you to defend higher up the pitch. Conte's (and Allegri's) 352 required the counter attacks to come from slow players down the middle or from wing backs who are attacking from positions that are too deep while defending. Against the good sides, we sit too deep and don't have the pace to threaten them on the counter. The change in formation changes things somewhat. Although I think if the formation evolved and allowed Coman and Morata to be part of it - we would have true pace on the counter in conjunction with the vertical runs from Marchisio and Pogba.