Atalanta performance was also not very impressive in Serie A last season, but in the Europa League they were able to show their best side by beating the two strongest candidates to be champions (Liverpool and Leverkusen) before finally confirming themselves as champions. Atalanta may be able to spring a surprise in the UCL, but Arsenal will not be easy to beat in their current form.
All teams always try to win in every match, but differences in quality and other unexpected things can change the outcome of the match. The new format is a bit tiring because the number of matches will increase, the coach must be able to keep his players to always maintain their best performance so they can qualify for the next round.
I don't mean to underestimate Atalanta, but when compared to other clubs in the Champions League, there are several clubs above them in terms of strength, including Arsenal in my opinion which is slightly better than Atalanta. However, they are still a club to be reckoned with and anyone who faces them must be careful.
Last season in the Europa League was a real example when they managed to beat the strongest candidate to become champions. Almost everyone was not on their side, but in the match they were actually able to become winners, including beating a club that was unbeaten in all competitions last season.
On the other hand, the difference is that because the Champions League is different from the Europa League, here they will get more pressure, so they must be ready for that by improving their mentality.