I actually think it could be the more offensive teams that can hurt us. We shouldn't have a problem shutting Chelsea out until we get a goal. After which they open up and we can score at will while still keeping them out because they just aren't that good offensively.
A team like Liverpool or Man City, on the other hand, has the tools to beat our defense and put us in a position where we have to attack, thereby exposing our rather lackluster offense. Even if we score first, they can certainly pull one (or two, or three) back. Against Chelsea, we score first and the tie is done.
Honestly, this whole idea that parking the bus is some super secret special formula to beat Barca is bullshit. Bayern (by far our greatest defeat since 2008, and it's not even close) didn't park the bus in 2013 (we didn't even get out of our own half in that game
). Nor did PSG last year. Chelsea 2012 was pure luck, could easily have been 10-0 on aggregate for us. Atletico 2014 and 16 were defensive but not to the extent of Chelsea 2012. 2014 especially, they actually looked best in the first half of the second leg when they were pressing and attacking and overrunning our midfield and hit the crossbar 4 times. Juve last year, again, defensive but what made the difference in that game was they were more clinical than us.