Atalanta was able to win Europa league does not mean that they are strong in UCL. Inter is more stronger and experienced than Atalanta and for that reason I don't think Atalanta will be able to pass the group stage because I don't see them having that strength.
There are big favorite clubs in UCL that will always prove themselves as big clubs by winning the underdogs and will go far in the league.
I agree with you. UCL and UEL are on different levels. Atalanta were impressive in the Europa League and even managed to beat Liverpool who often play in the Champions League. But now Atalanta are really playing in the UCL. I am not sure Atalanta will be able to do the same as they did last season. All the top European clubs are in this competition. They will always be motivated to win the Champions League.
And Atalanta in the UCL cannot be underestimated. because Atalanta also has a lot of experience at UCL. It's just that the competition this season is much more difficult because the number of teams in the UCL has also increased. And a changing system can also be another challenge.
The competition here will be very tight. I doubt Atalanta will make it out of the group stage. The new format in UCL will make the team that collects the most points will advance to the next round. And since all the teams are in the same standings I think Atalanta will still have a hard time getting a place in the top eight. But I see them still having a chance to make the playoffs.