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Newcomer clubs cannot be underestimated, that's true but most of them are no more than memecoins that just follow the hype. I don't believe in them as underdog teams because every season there will be one of the many clubs competing. If Atalanta is used as an example, maybe this season there will be a team name that will be an underdog like Brest. This club from France got a total of 6 points because they were able to win against two teams by scoring 6 goals.
Apart from Brest, there are still other teams that are not considered that might be able to follow the teams that were once underdogs in the following seasons. Rather than guessing, it's better to just follow who they are with the new format.
No team can be underestimated in the Champions League, but the favorite team can always be the favorite.
Favorite teams benefit from many factors, including squad depth and experience. The favored team can beat the favorite team, but this happens very rarely and only occasionally.
The underdog team could qualify for the round of 16 or the quarter-finals, but it takes more hard work to reach the semifinals and above. Favorite teams are more likely to be in the final than all underdog teams, in fact most of them will only end up in the playoffs or the last 16. But like you said, we'll see.