Atalanta seems to have the feasibility to compete in the champions league, the title of winning the Europa league is indeed worthy of them last season, the competition against Atalanta is a match that is indeed very undefended, yes the penalty that was wasted was an opportunity that was wasted but still saved by Arsenal.
The Champions League trophy by Arsenal seems quite difficult, for some reason in the Champions League Arsenal are not so good, as far as I know that Arsenal have always failed even to the top point of getting a chance to win, and they have never been a favorite winner, Manchester City, Real Madrid and Liverpool are always expected in the Champions league.
It is way too early to draw any conclusions whatsoever at this stage. I think Atalanta is a team that can come up with some surprises here and there, but they do have their limits. The Europa League win against Leverkusen was quite a surprise already because who would have thought that they win that final in such convincing fashion. But part of the truth is that Leverkusen on that day just didn't perform the way they used to perform the whole season.
But as I said Atalanta has limits and I think Arsenal is stronger than them, still though the Italians have to be taken seriously. However, the Champions League win is probably out of reach for them.
And for Arsenal, tough call. I think they don't have this final little piece that is needed to take home the matches against the top teams in the knock out rounds and especially in the semifinal and final.