Make your decision mate. Should they be underestimated or not? 
Chelsea are really highly favoured for the trophy. There is nothing wrong with that considering the quality difference between 2 teams.
There is only one handicap for Chelsea here. They played a Champions League decider match in the Premier League 2 days ago. The fatigue will be their biggest enemy. If they deal with it well then I don't think there will be a surprising result.
It's obvious the difference in quality but we never know where the surprise will be, will Chelsea lose? That would be normal in my opinion. Chelsea are really highly favoured for the trophy. There is nothing wrong with that considering the quality difference between 2 teams.There is only one handicap for Chelsea here. They played a Champions League decider match in the Premier League 2 days ago. The fatigue will be their biggest enemy. If they deal with it well then I don't think there will be a surprising result.
The Premier league decider made Chelsea more determined to win and now they're doing that job again this final so I think Chelsea will win this one.
I wouldn't even be surprised if Chelsea lost dramatically.
There is difference in squad strength and it's obvious that Chelsea are stronger than Betis but that doesn't rule out the possibility of Betis beating Chelsea. This is a final and all teams will definitely come prepared. Chelsea have had the mindset of qualifying for the Champions league or Europa league even when they didn't have hope in making top 4 they were determined to win the Conference league and get a spot in the Europa league at least but to their surprise they made top 4 and now in the champions league now they will want to win the title to have a more successful season.
Chelsea could lose as you mentioned but I would be surprised if they do.