First of all, why keep going with the 'black-and-white' assumption? Just because someone says "everything isn't so rosey" doesn't mean they are saying "everything is awful".
Secondly, most or all of what people are saying in response to people being all-out positive is just providing an alternative view. So if someone says, "I can see no serious opposition and I'm very confident we will win" (words to that effect), then someone else will inevitably say, "but what about the players we've lost"?, and/or, "you can't just write off this or that team".
This does not mean, as the black-and-white view seems to suggest, that they are saying "the opposition are better than us, they have the players and the tactics in place to beat us". It means, "consider the possibility that this or that may happen, and that we are no longer the squad we were, so we can't base predictions on last year's All-I" (for example).
Then as regards your favourite question, "who's the serious oppo"? - This is answered in the context of the above, in other words, that how good we are/potentially could be, compared to how good other teams are/potentially could be, is a relative thing.
To pose a scenario in practice, Mayo took Dublin 2015 to a replay, that was Dublin with almost a full pick of the best players available. And also a very hungry Dublin squad who badly wanted to put right the loss in 2014. And who hadn't been celebrating an All-I since 2013.
Mayo play Dublin in 2016, for the purposes of the mathematical equation, let's assume Mayo are the same as last year. Same players, same experience, same hunger etc. Let's call the changes in the Dublin situation X. And Galway are the 'x-factor' team. And there's twelve teams left in the competition. Gooch is back to full fitness etc, and most people agree Tyrone are the new favourites. And Connolly is likely to get sent off.
This year the equation looks like:
D - X - DC / M - A(i.D_0) + G / 37 + MH / 1 = KA* / 26 x 1 / 12