Arsenal are the Jonny Bairstow of the CL.
They often look great domestically in an environment where they are familiar and comfortable and their skills are generally better than the opposition, but take them out of their comfort zone against more capable, skilful and savvy opponents and they get found out most times. The flaws in their game are isolated - some lack of pace and mobility in defence, insufficient quality in central defensive midfield and a lack of closing down/pressing/general awareness when the opposition have the ball.
For the past 10 years under Wenger, Arsenal have only been capable of reaching a certain level - which has perenially been good enough for 3rd/4th in the PL and last 16/last 8 of the CL at best - but not beyond that.